It looks like the tree construction and internal to external mapping is not working 100% well.

See the attached tree:
sibling_subtrees.txt

It's fairly simple, it executes twice the subtree "Subtree1", which checks that a param is 1.

The problem is that the second Subtree execution fails, because it can't read the input port of param.

I started digging and got up to the point where the blackboards are created, and it looks like the parent of the blackboard of the second subtree, is the first subtree's blackboard, not the root node's blackboard.