I would want to see more info on what you are building before I could offer up any suggestions. Like are you going to bolt it to a wall or hang it between posts?
I've done one 14 foot x 2 foot
sign backed with dibond. It's too soon to tell if it was a good idea. It's been up for about a year and it seems fine.
I glued the dibond and HDU so that the dibond overlapped the joint in the HDU. It was nice and solid.
I'm more concerned with the thing expanding and contracting with the weather than the glue failing.
Danthonia Designs does all it's HDU
signs with a pvc foam board (Sintra) backing. They use some sort of 3M double stick tape to laminate the two together. I can tell you that urethane glue is way stronger than HDU so I have no worries about using it to laminate HDU to Sintra.