Sorry, but that will not work either. The restriction is that no volumes may be mapped to additional FA ports while the RP splitter is replicating data. This applies regardless of how the Masking Views and Port Groups are configured. Your only option is to stop replication, map the additional ports, and restart replication.
Current have a masking view that is used for a group of VMware hosts giving them access to their VMFS volumes that are all tagged for recoverpoint.  The masking view unfortunately contains a port group which only contains ports from a single director vs. both directors.  What I've learned is that if I attempt to just add two new ports to the port group that are on the other director the vmax will complain because there are active copy sessions (recoverpoint).  I'm wondering if to get around this problem and not stopping replication everywhere if I can just create a brand new port group, shutdown all VM's running on those hosts, and swap out the port group currently in the masking view with four ports from one director and replaced it with the new port group that contains 2 from one side and 2 from the other.  In my example recoverpoint is masked to all the ports so I'm good there.  Will anything prevent me from swapping out the port group similar to my inability just to add two new ports to the existing port group.
VMAXe Masking View (Recoverpoint tagged storage group)