Do you not see how incredibly limiting to gamestyles this would be?
If your group likes restriction, you can agree on some, and 3e-style multiclassing potentially contains 2e-style multiclassing if you just apply the proper restriction, in this case "max 2 classes, and must be max 1 level apart". Here's your 2e-style multiclass PC within 3e-style multiclassing framework.
We used this kind of restrictions in our 3e games very often, because we also weren't fans of heavy multiclassing driven only by "combo" powergaming. We switched several time between different combinations of max number of classes, max level difference, racial restrictions or benefits, and XP penalties.