Assuming you cannot use your cells in Mexico. Assume you have no reason to both be on the phone at the same time while in Mexico. Assuming you realize that every call to your cell that is forwarded will be using cell minutes.
If someone calls your cell you want the call to ring on the OBi in Mexico. You want to be able to make calls from Mexico to the states.
Buy two OBi 100s. Set one up in the US, one in Mexico (but first setup the device in the US). Setup the one in Mexico with two GV numbers. Forward your two cells before you arrive in Mexico to the two GV numbers. When someone calls either cell while you're in Mexico, the OBi rings the house phone. If you're on the phone and someone calls either cell, it goes to GV voicemail. You can call OBI to OBI and then make free calls in the US either with a GV number on the US OBi or your landline (in which case you need a 110 in the U.S.).
I don't have familiarity with GV in Mexico to know if you can make a call on it to the U.S. for free, that's why I said OBi to OBi.