1/ May I install both versions of MemoQ on the same computer?
2/ May I reuse my custom MemoQ 6 locations (for new TMs, new TBs, new projects, and so on) for MemoQ 5?
I don't know. I'm using the default locations chosen by memoQ and it works fine.
3/ Is there any particular issue known for using both versions concurrently?
Not that I'm aware of. If you open in memoQ 6 a project created in memoQ 5, a new project will automatically be created for you, with "[V6]" added in front of the project name. You will therefore have two projects: one that can be accessed in v. 5 and one that can ve accessed in v. 6. It's not an issue for me, but maybe it is for someone else.