Hello ckitchen, welcome to the Unreal community and thank you for posting this question.
Here is a neat thread where the members go over how they would implement a sub behavior tree. user Ogromord talks on how they would use blackboard keys for things one would expect in the in-engine tasks/decorators.
If you would like, here is a brief C++ video on nested behavior trees.
I hope this all helps.
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Your BehaviorTrees have two different Blackboards as their roots – those two blackboards do need to be compatible with each other. Not sure if that’s the problem or not.
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how do you make them compatible? Having the same blackboard makes them compatible but is there another way?