I'm actually a B52 fan for kicks. I work with a different artist every gig, so I like the reliability of the 52. It sounds the same almost any drum you stick it in, given this isn't always a good thing, but I find it really cuts down on
sound check time if most of my board can be
preset and what not.
I've never even considered using a MD421 on a kick, I always used them for toms. I never thought they had a big enough capsule to totally get the lows out of the kick if I already had another large
diaphragm mic lying around.
As to double micing. Never really had enough mics to
play with it too often. But in my experience it's a waste of an input. All the lows are caught by the kick mic as well as lots of the beater, but there's also a crap load of bleed through the toms and overheads.
One experience I've actually had alot of success with is the four mic kit. Two overheads, a snare/hat mic and a kik mic. I even place the kick mic a good
foot away from the drum sometimes if it's a blues or folk concert.