Yup, I use a handful of film cameras fairly regularly. I still use the second-hand Canon AE-1 and A-1 I picked up as a teenager about fifteen years ago. I have a pair of Mamiya 645 Pro TLs that get a regular workout and a couple dozen cheap collectable cameras.
About a year ago I posted a picture of many of my film cameras and it’s one of the most popular things I’ve ever pit up here.
As far as multiple exposures go, there are basically two ways to do it. Some cameras have a double-exposure lever or button allowing you to shoot over the same frame of film as many times as you’d like. If your camera doesn’t have that feature you can usually fake it by depressing the little rewind release button on the bottom plate as you crank to the next frame. The other way to do it is to shoot an entire roll of film, being careful to leave the leader hanging out a few centimetres after you crank it back in to the canister. Then you just load it back in to the same or a different camera and snap away.
I’ve been using my Fuji X100 for the majority of my pictures lately, but I also have the original 5D. The X100 is a great second camera and perfect for carrying around everywhere, but even after two firmware upgrades it’s still awfully quirky and a bit sluggish. If you’re only going to buy and use one camera, the X100 isn’t it.
The 5D mk II is old, and the technology inside of it - specifically the autofocus system - is even older. There’s a new model on the horizon, so I would wait a bit if I were in your shoes.
I love cameras and I’m fascinated by gear, but in all honestly it doesn’t really matter what you use. Some of the most enduring photos have been taken with very modest equipment. Lenses do matter, sure, but almost every decent camera today can do an awful lot.
I’m a big fan of the micro four thirds system, so take a close look at the Olympus E-P3 or the new Panasonic GX1. Both are great little street cameras, and there are dozens of great lens options for that system. You’ll be getting the small size of the X100 along with the relative speed and versatility of a DSLR, though their sensors can’t quite match the low light performance of a full frame CMOS.