Mark items far away from each other as grouped (connected)

I have a tree structure with items from different trees that I need to "group" (or really, mark as grouped).

Like so:

It is mandatory for the user to select a few of these items to carry on to the next step. Some items are mandatory, like item 1 in group 1, item 3 in group 2 and items 4 and 8 in group 3. And then there are items that the user can chose between, but one of them is still mandatory. Like the can chose item 3 in group 1 OR item 2 in group three have to be selected and item 1 in group 2 OR item 3 in group 3.

How to make this visually clear to the user?