To be clear, just the 2 - 100A panels?
2nd floor is the sub?
If so, how many conductors to the 2nd floor? 4?
just add a ground bar to the sub,relocate the grounds to this bar (drill and tap,this GRD bar.)
If 3 conductors,not sure about the CEC, but you can pull up a GEC from the main to the sub.