If more than one employee requests the same unassigned shift, the Pending Trades page lists them in the order received, where first request was sent first and all duplicate requests appear below that in the order received.
Note only one of those requests (the first one you approve) will be assigned and all other employees who requested that same shift will be sent a denial message that their pickup was not approved.
When employees look at their Tradeboard and click any open shift, they can see if there are already other employees asking for that specific shift.
So if shifts for the same time and Position are available, they should submit their pickup request for a shift that does not already have a pending offer to increase the likelihood that their request will be approved. It might be useful to remind them of that.
If you wish to allow both employees to work at that time, you would have to edit or create another shift for the same Position, date and time, and then assign one employee to this new shift and approve the other employee’s pickup for the original shift.