in my experience, this is how you do collabs--one person starts the project emails it to the other artist who finishes it and you both post it. that's really best for drawings. digital remixes of traditional art or of other digital art are good. line art that one person draws and another colors, whether printing it out and coloring it in with physical media and scanning/photographing it, or coloring it digitally is also an excellent way to collaborate. anything that is designed by one person and executed by another works, too. the main thing is for two artists to get together and create something they both work on.
one thing i would like to