I do this often. Get a Sharpie and mark the film leader. I usually write "go to" and the frame number i was on + 1 and the camera.
So if I was on frame 11, I would write, "FE / Go to 12." That way, you avoid possible overlap.
Of course, you need to be consistent when loading film. In fact, be consistent in everything that you do in life.
I have done this a few times in the past, and it works well enough. But be sure to mark your cassette very clearly. To be honest, the potential for trashing photos is not insignificant.
If you are shooting different types of film often, then you should consider an extra camera body. I know it is not without cost, but it will save you a lot of effort and potential disasters. Way back, I shot only one type of film - Velvia 50, and was happy with one camera. Nowadays, I will find that really difficult. I could shoot B/W or different film speeds with much more freedom once I augmented my FM with an F3. I am sure you will find an FG, EM, FE, FM10, FE10 or similar body for next to nothing if you really look hard enough, and that can be used as the backup body for the type of film you shoot less often. In South Africa such cameras are practically given away.
This is a big reason for my changing over to medium format. I wanted to be able to swap films mid roll and I didn't fancy spending the kinda money that Rollei 3003's go for so I bought an ETRS.
My 35mm stuff has relegated to the drawer since I bought it.
The other option I used to do as well was to have two cameras, one loaded with the slower film and one with the faster. I just couldn't deal with the hassle of swapping films mid roll and the extra chance of blowing the films due to light getting onto it.
I have also used it to rescue a film after a camera broke mid roll.
Most recent case was a Pentax ME-F which appeared to work perfectly (I fired the shutter with no film about seventy times with no problems) then developed the non-cocking shutter problem about six frames into the first roll. As all the M series bodies (apart from the MX) use the same spacing between cartridge, shutter and winding drum I was able to swap it into an ME, fire the shutter with the lens cap on to get past the already exposed frames, then shoot the rest of the roll as normal. Aside from three or four frames which the ME F had spoiled while expiring the rest of the roll was saved.
Great advice, thanks so much.
My other question is this: I have a few rolls of film (first two I ever attempted to shoot) that i opened up the back of camera on about 3 photos in. Could i still use the last 20 exposures left on these rolls or is everything lost when you open up the back?
The only potential loss is the film that is already on the take-up spool. So, yes, you can use the rest of the roll with a high probability of success.
That's great! Saving $5!- so if I was on exposure #5, what # would you estimate is safe to start on?
6, but to be absolutely safe go to 7.