I've actually co-run a one-shot this way, as the second installment of our "ridiculous campaign." Set in Sigil Prep, it involves the change from 3.5 to 4e...the next "curriculum change." The story involves making peace between the two, which can only be accomplished by going on a quest into the dungeon where all the old curricula died (except many 2nd ed characters now work the cafeteria). Eventually, our mixed 3.5/4 6th level party fought 1st ed monsters. I made approximations between saves and the various non-AC defenses, and basically ruled that any given attack uses the attacker's ruleset. The most challenging character was a schizophrenic cheerleader/bookworm aka 4e bard/3.5 wizard. Some of her magic items were merely nice looking depending on her current personality.
For practical reasons we didn't do a lot of fighting, but at 6th level the two systems had vaguely similar levels of attacking and defensive power. A serious comparison would be really difficult, but I think one could be made to work around this level. Much past that in either direction, the system maths (and spell abilities!) diverge too much.
For practical reasons we didn't do a lot of fighting, but at 6th level the two systems had vaguely similar levels of attacking and defensive power. A serious comparison would be really difficult, but I think one could be made to work around this level. Much past that in either direction, the system maths (and spell abilities!) diverge too much.