We have a web-based game for two players, which we offer via Amazon Mechanical Turk. For each game we need two players that will enter simultaneously, or at most 1 minute apart.
Using TurkPrime.com it is possible to bump a hit by using the "Restart" feature. It closes the first HIT and opens a new HIT excluding all workers who completed the first HIT. The effect is that it bumps the HIT to the top of Mechanical Turk when sorted by date.
With TurkPrime you use your own Amazon MTurk account, but they have an option for requesters with no MTurk account.
The only surefire way I know of to bump a HIT is to create different HITs of the same HIT Type and add HITs to that type. That bumps the creation date of the entire HIT Type for sorting purposes, which is what workers often search for. See my projects which use this approach:
The other way to get concurrent workers for your HITs is to post on worker forums.
Generally, if you are going to do things that require more than two workers at the same time, you need to establish communication with the workers and schedule something in advance.