You can buy a reversed servo from places such as Servo City.

If your radio doesn't explicitly have a function for dual elevator servos, you're likely going to run into a problem when mixing an auxiliary channel to your elevator channel: trim. Trim changes you make to the elevator channel won't be properly matched to the auxiliary channel.

I suppose another option would be tying the two elevator pushrods together and using just one servo (that'll be strong enough to meet the torque requirements). Depending on the layout of the model, this might be very difficult to do.

Having to move up to a better radio is often just one of those things that comes with running a more complex model. It really would be the best way to address the problem if your current radio doesn't have the necessary capabilities.