I've checked the manual. They have list for approved and tested memory cards to be used with D90 (page 242). With Toshiba and Panasonic, it can go to 32GB, but for Sandisk they only listed 8GB (not sure why, perhaps at that time SanDisk only have 8GB card?)
My understanding is that the D90 does not support SDXC. I don't think you would want to be frequently moving the memory between the bodies anyway. For proper operation, you would need to format it each time you do. Given the price of the D7000 in relation to the memory, getting a few dedicated memory sticks is not an expense worthy of much hand wringing. My $0.02.