For your 1st question of “JV- legally independent – employess be shared without conflicts”,
let’s call them partner companies: A, B and their JV: C.

Seek legal help on the JV agreement.
There will be conflicts if aspects of the JV are not defined properly.
Besides the legality, trusts, relationships between two companies, some aspects need to be considered and spelt out between the two companies:

1. Defined responsibilities, roles of the two partners.

2. Assets (and liabilities) owned by JV, e.g. equipments, materials.

3. Staffs headcount under JV

4. IP belongs to JV.

5. How to bill JV if JV uses assets belongs to mother companies or vice versa. For examples, JV uses equipment, materials, staffs, IP of mother companies. You need a system to track, for e.g. machine time, amount of materials used, man-hour of staffs, pro-rated cost of parts. These can be “tricky”.

6. Commitment of staffs: this is tricky if staffs are shared, for e.g. staff doing both project in A, C or B, C.
Doubts will arise when project stalled in JV. It’s best to have staffs headcount under JV and dedicated to do only JV project. If that is not possible, at least staff dedicated to JV project for a period of time and replacement when he is pulled back to A or B. Guidelines need to be spelt out for shared staffs.

7. Confidentiality and non-use and non-disclosure of JV’s IP, business secrets to staffs at A, B are very important. This will be a potential problem if share staffs between A and C; or B and C. These staffs have to sign confidentiality, non-disclosure, non-use agreements; companies have to sign too. This may be tricky, for example a staff doing project in JV, may learn of knowledge that can be implemented in A or B. Management should spell out what are acceptable and what are not acceptable.

8. Defined roles in decision making, line of commands etc..

“The JV would form and dissolve around specific projects but the framework would be in place to roll the JV from project to project. “ This part of your question adds to the difficulty, especially project-based, framework..

I would like to recommend you to the concept of “Open Business Model”, “Open Innovation” by
Center for Open Innovation University of California, Berkeley; Henry Chesbrough.

Resources on “Open Business Model”, “Open Innovation”
His books on “Open Business Model” (1 book), “Open Innovation” (2 books) are insightful.

One of the “forerunner” on this business model is Procter & Gamble’s Connect + Develop. (mentioned in the books)
You can see how they do it in a project-based framework from its website, Procter & Gamble’s Connect & Develop.

They shows their “needs”, “assets”, “interested in collaborating with you — with our IP, yours or both — on packaging, design, distribution, business models, marketing models, consumer research methods, trademark licensing, technology research (our R&D efforts are focused on 150 areas of science) and more.” in the website.
Check their P&G Bioscience, P&G FutureWorks too.

Their P&G’s Success Stories shows many different types of examples, varieties of cooperation, through licensing, capabilities agreement or JV.

Hope these helps. All the best to your JV.
Hope you can share your insights along the way too..

1)
There is no clearly defined business structure that will fit perfectly with what you are trying to do. I refer to what you're doing as a "competitive partnership". You need a custom legal document that is made up of 3 components: Things that protect you, things that protect your competitive partner, definitions that outline what the partnership entails. If this partnership will likely result in a large increase in volume (of production, sales, etc), spend a few extra dollars and have the lawyers define what the partnership does not entail.

2)
I have partnered with a company that competes directly in the same industry that I work in. We directly compete with each other. We know what we're good at and what the other company is good at. We've decided that we'll both make more money if we can use the strengths of the other company to compete on a larger scale. On a small scale, this saves me from hiring a specialized employee and on a large scale, strengthens my company by controlling direct competition.

For us, it seemed simple at the start. We simply defined geographical regions where our businesses were already rooted. The contract was written to say that we needed direct and written permission from the other company to market or solicit business in those areas. If a customer comes to our business from one of those areas we are to direct the customer to the other company unless the customer already has a closed account with the other company.

Confused? It is confusing. Even simple "split" partnerships need to be carefully defined. You need a lawyer. If you have anything more than a very small business, it's going to be a good idea for both of you to hire independent lawyers who will work in your own independent best interests and form a contract that "meets in the middle".

Some tips from my experience:

You need to trust the other company and they need to trust you. Beyond contracts and lawyers, if both companies trust that everyone is working toward a common goal, things will be a lot easier.

Open information is important. Communication is key. While you need to protect what's valuable, hiding information or obscuring facts will lead to large problems.

There must be enough work for both companies. If you already find yourself regularly competing for customers, a competitive partnership may not work. Someone is going to get hungry and trouble will ensue.

You will be associating yourself with the other company. Often competitors speak disparagingly of their competition as a way to elevate their own service. When a competitive partnership is formed, you can no longer do this. It's better to say nothing about your competitive partner than anything at all. You need to be proud of the other company and your relationship with them.

Last, meet regularly with the other CEO. For me, this is as simple as an occasional friendly beer where we talk about what's been going on. It's actually quite nice to talk to someone who understands what you're doing and you don't need to "pitch".