Depending on the ISPs yes, it is called bridging.
However it depends on if the cap if using the same ISP for both modems is applied at the modem or at the node/block. Most ISPs configure this cap at the node/block. http://www.wikihow.com/Bridge-an-Internet-Connection
Alternatively you can also buy a switch w/wireless that supports bridging, they usually also support failover/load balancing, but can be rather pricey as they are marketed more toward businesses/enterprise as high availability, such as a Dell Sonicwall, that supports Multi-Wan. http://www.sonicwall.com/us/en/products/NSA_220.html#tab=specifications Which is what I use currently.