You would want to have a linear fade for in and out, not a gradual one fading in slow, then fast till 100%, and out in reverse, which is sometimes also the case.
Note that for the best transition, both sounds should be equally loud. If one of the ambiences is not playing the same volume (even if it both says volume = -5 db (or whatever value you can set in your program) the actuall sound can be soft or loud at that point.
So you will want to instantly mute the sound and make sure they're qually loud at that point.