You can do it via multilink PPP on most Cisco routers. Here is a link for this:
http://www.cisco.com/en/US/customer/tech/tk713/tk507/technologies_configuration_example09186a0080093d69.shtml
However, I prefer using an inverse mux such as the DL-3800 by QuickEagle. With these, both T1's are presented to you as if they were one. You can usually run up to 6Mbps off a serial interface (ie: about 3-4 T1's), above that you would need to use a HSSI interface. I've used these with great success a few times...
I would avoid putting any other L3 devices (ie: routers) in your network. They cause problems with things like routing protocols and multicast and things like that. Our company was talked into going with a unit called a "tasman", but the company that talked us into it won't support it and it causes alot of problems.
Here's a good article on all of this:
http://www.cisco.com/en/US/customer/products/hw/modules/ps2033/products_white_paper09186a0080091d4b.shtml
