In reply to Ramblin dave:
Yeah, definitely do-able in a long weekend. Myself and a friend were up that way to climb on the west wall of mitre ridge last weekend. Managed to get over to very near the summit of beinn a bhuird in about 3 hours with bikes from invercauld. To me the south approach seems the most "sensible" particularly if you don't have bikes because the amount of horizontal travel is much less than the north approach.
In terms of places to kip when coming from the south you've got the "secret" bothy, though its not very far in on the approach so maybe isn't best placed if you want to do both munro's as you'll have to do some to-ing and fro-ing. also there's the smith winram howff at the foot of the dividing buttress between coire an dubh loch and coire nan clach of beinn a bhuird.
I'd only recommend the north approach (from tomintoul or I guess glenmore lodge if you wanted to be even more leisurely) if you've got bikes, or you simply want to make your trip very leisurely. The bonus of the north approach is your sleeping options are better. Faindouran is a pretty nice bothy, and is just a little way up the track from the wire bridge that takes you up the glen towards beinn a bhuird. Fordsof Avon refuge is only applicable if you're coming down strath nethy from glenmore lodge, it's also pretty spartan.