I'm in the uncharted territory now. My case may not apply to you.

For the part that I need the multiple products checkout, I decided not to use shopping cart. I just used view_bulk_operations module which is configured thru the views GUI. Then I provided my own action callback. So far the callback is only a stub.

The true test is going to be next part, which I haven't started yet.

I don't know if bulk operations can work with ubert cart, you can check out the view_bulk_operations module. It's very good for what it supposed to do.