I'd opt out of the 135mm... IMO, primes are too limiting in that dynamic an environment.
I'd go with a 24-105 and have maybe a 17-40 or a 17-55 on hand as well (for the 40D) once I knew more about the formation and how close you'll be.
IS would be a major plus since you're going to have to really drop your shutter speeds to get quality rotor blur. I'd start at 1/90 and work down from there. Maybe shoot some at 1/125 just to be sure you get some sharp shots of your subject if you don't have a lot of experience shooting really slow.
Are you familiar with both pilots?
Do they have formation experience? F.A.S.T. Cards?
Time of Day? Golden Hour light applies to aviation subjects plus the air is smoother at sunrise and sunset. The one caveat to this is the fact that a helo rotor is parallel to the ground so to get good glint off the rotor disk, you may want the sun a little higher in the sky so you can get a good angle of incidence toward the sun. As with fixed-wing subjects, flying a racetrack pattern can help you get a variety of sun angles and lighting conditions.
Thoroughly brief every aspect of the shoot.
Prepare a "shot sheet" that you can give to both pilots so they have an idea of what you want to capture. Storyboard the shots if need be. Consider inside and outside turns and if your pilots are comfortable with it, make turns both ways with your subject co-alt, below and above you. Helos often have both eyebrow windows and glass at their feet so visibility is pretty good.
Brief your Join Up, Separation and Recovery (back to base as a two ship or individually?), headings, altitudes, frequencies, and also discuss hand signals in case the radios fail.
"Knock It Off" procedures for safe deconfliction in case you lose sight of your subject. (Losing sight should be an automatic knock it off) Talk to your pilots about what to do if the subject ends up "belly up" to the camera platform. If you see their belly, they can't see you so both ships need to know what the other guy will do if this happens.
Wear black and tape over the camera logos so you don't throw reflections. Consider thin black gloves as well since your hands are also possible sources of reflection.
Advise your subject pilot(s) to keep their glare shield free of clutter, make eye contact with you at all times and smile so it looks like their having fun!
If at ANY point before you fly, something doesn't feel right, walk away! If ANY part of the shoot starts to get squirrely, call a knock it off and go home.
No picture is worth dying for!