Hi @sumarao9: When you say 2 different versions of PEGA to be merged to single 8.x instance, basically you are trying to merge 2 code base / 2 DB into single code version / DB. What I would suggest is either of the below.
Upgrade one of the application to the latest version of PEGA first and deploy the other code base to the latest 8.x version.
Upgrade both versions to 8 and do a code sync
Few impacted areas which you need to be careful:
Conflicts to be verified between both versions of your source in terms of organization category rules and data, classes, rulesets, access rights of users
OOTB table overrides to be verified
DSS and other data instance conflicts
As a alternate, check if process fabric can solve your problem of merging.