Allow participants to have more than one study in their queues
Anon
As on MTurk, accepting one study shouldn't preclude a participant from accepting another (with the understanding that the participant would complete the studies separately from one another).
This will allow participants to maximize their earnings where they otherwise would have lost out on either or both studies and will prevent participants from canceling participation midway through a study in the hopes of securing a spot in a higher-paying study.
Pea Kay
Being allowed to queue studies affects other participants who are available to accept the study and complete it right away. It isn't fair to have one participant hold on to a study while another doesn't have any to work on.
Allowing a queue would be more likely to see a participant cancel their participation midway through a study when a higher-paying study shows up.
Participants should be paying attention to the study they are working on rather than worrying about getting another, better paying study.