Recently in an AD&D campaign a player lost his character and the DM decided to allow him to make a new character with the same amount of XP. After looking at the PHB he decides to go for Bard (which involves Dual Classing). We were 7th level characters when this happened.
To our surprise he was able to make a 3rd level Bard dual classed as a 5th level Fighter and a 5th Level Thief.
What happened is that the rough doubling of required XP to advance really worked in his favor. It required only a modest amount of XP to advance to 5th level in both Fighter and Thief and then to 3rd level to Bard.
The implication of this is that in AD&D 1st; Dual Classing isn't as onerous as it first appears. Because when you do it, it is likely that you will continue adventuring with the rest of the party. The XP award will not be what you getting when you were at low levels but whatever the party has been tackling i.e. deeper dungeon levels. Advancement through the lower levels will be a lot more rapid than what occurred for the original levels.
When it all said and done it is likely the dual classed character will only be a level or two behind the rest of the party. By that point it is likely he would have exceeded his former levels and regained his old abilities. So as crazy as the old Bard looked it doesn't look as bad as it did before.
To me the Old School Renaissance is not about playing a particular set of rules in a particular way, the dungeon crawl. It is about going back to the roots of our hobby and seeing what we could do differently. What avenues were not explored because of the commercial and personal interests of the game designers of the time.