Always a tough question to answer as it will depend on shooting style, lighting conditions, ability to use flash etc etc.
I'm not sure what the 1DmkIIn is happy with noise wise so that might limit when it can be used.
Do you shoot one body with the other as backup or shoot 2 at the same time with different lens options on each?
I would be happy using the 40D and 17-55 for a lot of the work then it would depend on the look of the shots I wanted. I would probably pop an 85mm f1.8 on the mkIIn if you wanted to use it in the church, it will give you a little extra reach and shallower DOF over the 40D with 17-55. If you are happy with the 1DmkIIn at ISO 3200 then a 70-200mm f2.8 with IS/OS would also be OK, if stuck at 1600 or lower then you would find a lot of venues where it is just too dark to make use of. I'm often at f2.8 and 3200 and still at pretty low shutter speeds.
I never really got the whole diffuser thing, if you are bouncing it's not needed and if you are straight on it is pretty much just reducing the power as your light source is still small. If you want something when firing straight in then just pull out the white card and wide angle panel and aim up at 45 degrees. At a wedding I don't have time to mess around with putting on and taking off a diffuser, and I'm not going to leave it on all day as regularly I want to create soft directional light from a wall back to the subject so need a lot of the power I've got. Check this link out, certainly creates more natural looking lighting than straight in the face or ceiling bounced
http://www.lightcraft-photography.com/Other/side-wall-bounce-lighting/12283790_sdqfh/1/876235592_RdxiQ
Shooting a wedding is all about being confident in your kit and ability (general and using the kit you have). It has to be you who is happy working with the kit you have to be able to produce the quality of work expected by your customers. I will say when I started out I had a 5D and 30D and if needed to work in really low light would use a 50mm f1.8 on the 5D and 85mm f1.8 on the 30D, it was fine for starting out, not ideal but got the job done. You have the 2.8 with IS so that's going to work in most situations as long as your subjects aren't moving but the IS won't help if they can't stand still
OK, so long answer, might or might not have helped.... LOL.