You should be able to do it but you may need the ISP involved you could try use MLPPP as 1 type of feature not sure if that would work fully with mpls though on provider side , if you cant use the ISP you can load balance across the 2 links using CEF , use equal default routes for both links and setup ip cef load-sharing
Have you talked to your provider about getting bonded sdsl , you may need a specific wic in the router to achieve this on your side though but i think its possible in some cases
Just an option to try anyway
http://www.cisco.com/c/en/us/td/docs/ios/ipswitch/configuration/guide/12_4/isw_12_4_book/cef_load_balancng.html