Robert - you can do two qwith only two forks but the birds will wobble on the spit as you do it. This puts some strain on your motor and can beat the birds up a bit as they will fall up and down agains the spit rod. If you jam them tight together to prevent this, give your cook some extra time as the birds will take longer to cook. I would definitely go out and get a couple of extra forks for your spit...
Regards,
John