Use drive with both XP and Win7

I just bought a 3 Tbyte USB drive.  I’d prefer for it to have 1 partition, and to be able to back up both XP and Win7 machines to that one partition.  But as far as I can tell, there’s no way to do that-- different drive formatting is required for XP and Win7.  So my next choice is to have two partitions (each less than 2 Tbytes), and back up the XP machines to one and the Win7 machines to the other.  But I haven’t been able to find directions on how to do that-- the sequence of formatting and partitioning.  Can someone point me to directions on how to do that (or, worst case, tell me it can’t be done)?

Thanks.

Maybe this will help 

http://wdc.custhelp.com/cgi-bin/wdc.cfg/php/enduser/std_adp.php?p_faqid=3865  I believe WD has a format tool and you need to select fast format and there is an option about XP if I remember right. Find your drive at WD and it shold be in the downloads.

Joe