Two Calendars for two members of staff, who gets assigned when they are both available on the same day

Evening all,

As I have two members of staff, when clients book they currently get the option to see the availability/book a full day with either  'any available', 'staff member 1', or 'staff member 2'. If the client chooses 'any available' to see the availability and then completes the booking, does it assign that booking to either staff member 1/2 at random, or can this be set?

I hope i've worded this correctly and it makes sense!

Thanks,

Richard