In addition to import geometry function, you need to make sure the initial and boundary conditions. In some instances, if you have one model developed for simulation period different than the other model, then, all IC/BC are required to be updated accordingly.
I have gone through detail merging two HEC RAS procedures. We call it ‘stitching HEC RAS models’. If you have more than two storage areas and would like to use RAS 2D, condition becomes more challenging.