How to consolidate two web platforms with slightly different IA?

I need to consolidate two web products that have some similarities and many differences in labels and groupings. For round 1 testing, I plan to do open card sorting with content from both sites. I would test product 1 users, product 2 users and internal group with industry standards knowledge. I'm hoping that by triangulating with the 3 groups I will have enough insight to build an IA to test with TreeJack. Is there anything else I can do to accommodate both product users' mental models?