You've actually got a few items there I should address, but we'll start with the servos. If you mount the servos 'opposite' to each other and use a Y cable, they will move opposite (ie. Like ailerons). If you want them to also be flaps, you remove the Y cable, and plug the 2nd servo into an aux channel. At this point you either need a transmitter that will mix, or an onboard mixer. I don't know the mixing functions of your transmitter, but if you tell us what it is, I'm sure someone here can help.
Flaps: You say your reason for flaps is to 'slow down that fast-gliding plane for landing'. I'm going to attempt to get you thinking a little differently about flaps. Here goes.
1) Flaps do not 'slow down' an airplane. Flaps generally reduce the stall speed speed of an airplane. This slower stall speed enables you to increase both the angle of climb and the angle of decent.
2) Aircraft speed is usually adjusted through a combination of thrust and pitch angle. As you presumably already have the thrust at zero, you decrease the airplanes speed by increasing the pitch angle (nose up).
I don't know your pasture, so flaps may actually be necessary. Let me tell what I do in order to 'slow down' the airplane for landing.
First, I want to be in level flight at minimum power (that's 50% throttle for my Hawk Sky). Next, I want to apply full back elevator trim (don't have any more trim? This is why zeroing out the mechanical trim is so important). Lastly, I want to pull the throttle to zero.
Here's the key, slow flying airplanes need a much smaller turning radius than fast flying airplanes. Your plane should be able to fly slow enough that you can do a circling decent inside your pasture. I haven't tried it with my Hawk Sky, but I'd guess decent radius is under 50ft. How small is your pasture?
If you find the airplane is still too fast with full back elevator trim, try adjusting the the linkage to the point where you can give it full back trim, but the plane won't stall. You can test that at any throttle setting. Speed is speed, so if the trim setting stalls the plane at 50% throttle, it will stall it at 100% or 0% throttle too.
Hope I haven't confused you more.