aryoba is right - what you want to do is route requests based on source IP - not destination IP - so routing is not going to help here (unless as someone suggested, you assign two different DG address - and even then, can't do what you want on the PIX).
workaround: do it on the 2610 - connect the ADSL line to the 2610 and then do policy routing to send traffic either thru the T1 or the ADSL link
