What type of equipment do the T1s come in on, other routers plugged into a hub/switch on your 3rd ethernet segment? Does the switch have FlexWan (?) ports capable of handling T1s? Is this a metro type of connection (Ethernet from carrier)?
since you are NAT'ing and I'm presuming the 3rd interface is the NAT outside address if these 2 T1s are to 2 different ISPs then NAT'ing may be a problem depending on how your address assignments are. Need more details.
As far as load balancing, it is 1 ethernet connection from your 2611 to this hub/switch where these T1s are coming into if I understand correctly. You could put 2 default routes on the 2611 pointing to the IP address next-hop for each T1 and then you'll have 2 equal-cost routes. But this may not work depending on how these T1s are coming in.
Can you provide some more details on the connection?