Thanks for your comments.
The requirement is, business decided to Merge two roles into one.
Already these two roles has assigned with multiple users in an application.
So now we need only one role (RPF field).
So thinking about the best way to merge it.
In case in future if we have to merge two same field data into one (both are similar type); in same application for multiple records!!!
If you have 2 RP fields and you only need one and they are both inheriting from other applications, then, inactivate one of the fields and then configure the remaining to inherit from the same fields in the 2 applications.
If you have 2 manual RP fields, then you can inactivate 1 field, add the correct groups to the field population, and then either have to do a data import or data feed into the 1 remaining RP field.
You mention roles, which is different than a field.
So, If you want to inactivate a role and give that access to users that had that role and not the other, then you'd manage that how ever you manage provisioning.