Back in the bad old days, I did a really nasty thing. If you're familiar
with Chaosium's Ringworld RPG, you know that the great spinward ocean
contained islands that were 1:1 scale maps of worlds. I made an island chain
Glorantha, and let the interworld explorers set down there. Combined two
running games into one. If you're familiar with Niven's books, imagine a
Kzin trying to teach hyperdrive piloting skills to a duck. Luckily for
everyone, the Puppeteers had no natural ability to use magic, but the humans
did pick a few things up. The system's were compatible, and everyone (except
the duck) had a great time.
Joe DC