You would have redundancy (sp) if one quit the other should still work, giving you at least a bit of control.
You would also add to weight,battery drain, and just one more electric componet to not work right.

2 Servos are not needed on a 40 size plane. Most sport 60's could go with one also, however the trend has been 2 servos mainly for the use of flaperons, but if you cannot mix them in then I personally don't think it would be worth it. Unless you are planning on upgrading your radio.