I need two 12" extensions to install rear-mounted servos on my Somethin' Extra but all I have are 5 9" available. Can I connect two together as you do christmas lights or is that overkill on the servos and/or receiver?

You can... but you're adding more wire length than you need and an extra connector. If you're using standard servos, it's probably fine... if you're using high-torque servos, probably ought to get some heavy duty 12" extensions...

Every connector has a little voltage drop associated, so the servo will see less voltage and that means less power to the servo to do it's job. Probably not a big deal in your case.

More of a problem, more connectors are more places for failure. Unless you're using gold-plated connectors, it's more places to corode and cause the servo to act flaky. If it was me, and I needed the plane set up now, I'd cut two extensions and solder them together, I might even cut the connector off the servo and solder the longer leads in, removing as many connectors as possible. However, I trust my soldering, and you'd want to avoid having the wire unsupported and flexing at the solder joints as thet can break at that point.

If you use shrink tubing around where they connect, it will hold them in place better is all I can add to this one.

Decided to go ahead and get some Hobbico 12" extensions for Futaba radios. Relatively cheap so why not?

Make sure they have gold plated connectors. I think the Hobico ones do. I don't buy non-gold plated anymore, and will sometimes snip the leads off of servos that aren't gold and solder on new ones. Connector corosion sucks.

(btw, connectors that you unplug and plug often, such as removing a wing, generally won't have corrosion problems, but the ones you leave plugged in all the time can sometimes get flaky after a while, gold plating reduces this problem quite a bit. (I'd say "eliminates", but I hesitate to imply 100%)).

There are probably more failures due to using extensions than any other cause. Especially if the connection is hidden and not repeatedly plugged and unplugged to clean the corrosion off the connectors. If you are in a damp climate, this is even more possible. If you can, eliminate the extension by splicing in a longer length of wire as suggested above.