Ok, there is a knack to this.
Estimate where the first bus will stop. Work out where the back of the first bus will therefore stop, and stand there. When that goes by you, waive frantcally and the second driver will stop for you.
If the driver sees another bus infront of him he is actually supposed to stop automatically just in case, even at a request stop, so they will be wise to the possibility fo someone trying to hail them.
I am a sad bus geek - let me know if there is anything else I can help you with (short of fare evading tips, they are secret!)
Victoria