As others have mentioned, I'd look at renting the smaller home until you're ready to sell. You can look at doing a short sale, but I wouldn't recommend it as it's hard on your credit and I simply don't think it's good to walk away from a debt you can cover. Talk to a real estate attorney to see if a short sale is possible for you.
Assuming you're not going to do a short sale, you need to figure out how to come up with the difference between what you own on the loan + closing costs and what someone will offer for your home. If you don't have savings or other liquid assets available, then you can consider refinancing your other home. If you have a 401k, you might be able to take a loan from your own 401k account. There's trade offs and additional expenses with either of these options. If you don't have liquid assets, I like the idea of renting the home out for a while. If you can rent it out and get enough to cover the mortgage and all other expenses plus bank a couple hundred a month, then you can use the rent to give you time to save the $s needed to bring to close and the rent can even help build up this sum.