Forms are limited to the first level of datas.
I answered about that fea time ago. It is a simple question of logice and it cannot be sort it out because it is a non sense.
In your case, you can do it only if UserProjects and Projects are considerated as 1:1.
Means that UsersProjects is an extension of the Projects table.
Cook do not handle this fucntionality now. But you get a good start if you pre-build your requirements before.
Create 2 Layouts that you will merge afterwards.
When you save, you get 2 forms in one, and you will must split the datas to save the correct informations in the correct tables, using the correct ID value for the two linked items.
It is not a big deal, but it starts to be complex.
1:1, N1, and N:N are not so easy to realize.
In future, Cook will really rock on that !!