Will be getting tiger 2arf with os.46ax. Question I have is does anybody know how hard/easy it would be to put 2 servos on wing instead of 1.

Generally you should be able to widen the hole to fit the servos side by side. I would use smaller servos tho such as Hitec 225's. You can add a ply rail to make the shorter servos fit properly.
Generally, unless you are going to rebuild the wing so as to add the servos to the middle bays of the wings, its not really worth the effort with shallow ailerons.
I have never been impressed with flaperons if that is what you are after.

So is it better to build arf like it was designed than to add 2nd servo to wing?

Unless you have a specific reason for adding the second aileron servo, yes it's better to build it as designed.