you can have any combination of GPU make(amd/nvdia,etc) models(RTx/GTX/vega,etc) and brands(msi,asus.etc) BUT if u get 2 of the same, you'll be able to SLi later if u decide to convert to a solo PC down the line
side tip, if your going for a dedicated 2 user 1 pc i highly recommend getting 2 boot drives ( not partitioning one disk) and if u can get a server motherboard with 2 CPU sockets, makes it a breeze so each person gets a dedicated CPU....IF its low usage then 1 CPU should be fine, but still get 2 drives