Never try it, if it is NOT MIKE

[Patrick]

Traxxas says the titans and EVX can take 14 cells. The titans do get kind of hot and I don't think they'll last as long (not that there expensive though), but then it's more reason to go brushless. 14 cells on the titans is deffinetly more fun than 12 cells.
I don't see any reason why a UBEC wouldn't work if it's wired up right, but I don't think theres much point (unless your pushing it by using more than 14 cells or lower turn motors), because I think (I could be wrong though) that the internal BEC in the EVX only takes power from one battery and not both.
So it only has to convert 7.2 volts to 5 or 6 volts, not 14.4v to 5v, so not that much heat would be produced.