I have a couple of 555's.
Channel 5 is typically for something such as retracts. If your transmitter allows the use of channel 5 for something else at your discretion, cool.
Your statement of a typical wing having two servos usually doesn't require another channel. Just use a "Y" connector to connect both servos to the same channel, providing that the linkage is set up properly.
I have two servos on a few of my wings. In one case it was to experiment with Flaperons on a trainer. This required the use of another channel. This was dictated by the transmitter, and required a receiver that would accomodate this application. In this case it required the use of channel 6 for the second aileron servo. Channel 5 is still reserved for something such as retracts. I use it in this particular model to to release a parachute jumper for one of the other fliers at the field and in our airshow.