You don't mention which control software you are running, or much other details about your set-up. Mach3, for example has facilities to slave two axes together, so assuming you're not already using A, you could partner this up with Y so the two operated together. You need another motor, of course, and another motor driver - and hook this up through your existing BOB set-up (again assuming you have a spare axis route here). Hope your power supply has the necessary capacity, or this may need upgrading.
You'll see all sorts of threads on problems with slaved axes - I'm taking theoretically, because I've not tried it. Let's see if we can get a contribution based on experience.