This is a good point. I usually signal the turn before applying the brakes. If it's a left turn, I can be applying the front brake (I've got my brakes set up "moto," or the old Italian style, but that's another rant!), but for a right turn, I signal with the right arm, then stop signaling so I can get on the brake. I usually only use the front brake, but I'll use both on a descent, or for a hard stop, or if I'm driving the tandem. I would usually do all the signalling on the tandem anyway since my previous stoker was directionally dyslexic. I'd say "right turn" and she'd stick out the straight left arm...

Luis