Your DSL service is a point to point service from the Phone Company's central office to your one DSL modem, If you try to parallel a second DSL modem the DSL equipment at the phone company will not know which one to "hook up to" and neither may work. To share your Internet connection between the two buildings you will need to extend your Ethernet network between the two buildings. The best and safest way to do this is with a fiber optic connection so there is no copper electrical connection between the buildings.
The next best would be to use a bridge device that will let you run the Ethernet network between the buildings with a long copper link between. Black Box and others sell equipment to extend an Ethernet network between buildings.
http://www.blackbox.com/Store/Detail.aspx/Ethernet-Extender-Data-Only/LB200A-R2
The last option would be to run native 10mb Ethernet between the buildings on copper wiring.
With any of the copper wire solutions you must install station protection at each end of the wire to prevent a dangerous or possibly fatal situation. If one of your buildings were to experience a power surge or lightning strike, that voltage spike could travel through the copper wire between the buildings and cause damage to people or equipment connected inside the other building. The phone company mitigates the risk by installing station protection on each end of the wires traveling between buildings (even on cables buried underground.)
This can even be an issue if the ground reference between the building changes. (kind of like people who got electrocuted by touching the disk washer and refrigerator at the same time and there was a grounding issue with one of the appliances.)
That is why the Fiber optic is the best solution.
Your phone line between the buildings needs the station protection also.
Dave