(The TurkPrime exclude feature will exclude workers who completed one study from taking another subsequent study but not if both studies are run at the same time.)
Solution: The TurkPrime Survey Group (arriving August 25, 2016) feature allows a user to assign multiple studies to a survey group. Then, any worker who accepts any study in that group will be disqualified from accepting any other study ion that group. Your studies will be guaranteed to have a mutually exclusive group of respondents.
The Survey Group option is also a broader exclusion than the standard Exclude Worker option: Survey Group excludes workers who even only accepted one study from taking the other studies in the group, while the standard Exclude Worker option only excludes workers who completed the study but will not exclude workers who accepted but did not complete the first study from taking the second study.