We are planning to do concurrent development using two different Appian sites,

If I understand correctly that there will be active development on both sites, and if the sites will be in sync, what is the advantage/use-case to having two sites at all?

Adding to the above, you could go through the playbook topic that may be useful :
forum.appian.com/.../Managing_Multiple_Concurrent_Delivery_Teams.html

@Tom, Yes, we are planning to do concurrent development in two sites. What I meant Sync was ,when shared objects modified in both sites, we need to make them Sync periodically between sites as per Appian suggestion, and we need to merge two code bases into one site in QA to do functional testing.. We feel merging code base is challenge.. Both Appian CI delivery and concurrent development playbook topics does not suggest concurrent development using multiple sites...

@ Sonalk, we already looked at this link, It is suggesting to do "Coordinate and plan merge activities as part of a program-wide release schedule". Not sure the suggestion to merge generated XML files or Redo changes manually if we are using separate instance for development?

In order to 'merge' onto a single QA site, you can still follow the standard practice of application imports/exports. You can use the deployment automation tool for this if you prefer. There should not be any issues with this as long as no naming conflicts are introduced. For syncing objects you can do the same.

That said, I still don't see what the advantage to this approach is, considering you will have the additional overheads of syncing objects etc.

@Tom The advantage is we can save time if we do Parallel development using different sites. The actual question is, what is the best option to merge if same object is modified in different DEV environments.. Manual redo is the only option or modify XML and import ZIP to target environment or something else?

@ Tom Any suggestion, How is other Appian customer doing Agile development? Appian Continuous Integration playbook suggesting to do merge for User Case 3. Not sure it is XML merge or manual redo/import? please suggest? - https://forum.appian.com/suite/help/16.2/playbook/Release_Management.html

Note that for the Use Case 3 this is still a single environment. If the two teams are working on different objects, then there is no need to have them on different environments. If they are working on shared objects, then I think the overhead of merging updates will outweigh any benefits.

Even single instance or separate instance, I think merge / rebase to be done in GIT / SVN Repo for XML generated from export? IIs my understand is correct, Can we do manual merge in case of conflict during merge or rebase operation?

Yes, you should follow the steps mentioned in the documentation you mentioned:

forum.appian.com/.../Release_Management.html