Drum and drummer.
I think this is all gonna depend on the requirements of the song itself.
Record both kits as mono
I dunno, I've never done it before, but it seems like fun.
This is all gonna depend almost entirely on you getting two stellar, perfectly in sync performances.
Have you decided whether you're gonna record them at the same time or have one try and overdub himself onto the other?
Cancers makes some really good suggestions.
The key here is in figuring out if possible beforehand how you want the endgame to sound. I've been recording songs with two drum kits playing simultaneously (in the final mix) for a long time and to be honest, I've done it in different ways and sometimes it's come out better than I expected, sometimes it's been cluttered to the point of being an incoherent, indecipherable mush that you'd need three ears to hear properly.
It's really depended on the song in question. In the days when I recorded to an analog portastudio, the drums had to be mono tracked (I'd use a 4 way mixer but with a mono out) and on mixdown, they'd be panned at varying degrees left and right, not necesarilly equadistant. It depended on which drum track was deemed to be the 'main' one.
I've also done the thing of using the Roland electric drums (in this case, the TD6) as one drum and my then kit, a cheap Mirage as the other. But this was kind of an afterthought as by the time I had the Roland, I'd given the Mirage to my nephew. But for the few songs that carried this combination, the contrast is interesting as I find the electric drums had a thinner sound.
The other crucial question is are the two drums going to be playing at the same time, as Cancers pointed out. I've never done it that way.
I've also used the technique of having one kit played with ordinary sticks, while using chopsticks (yeah, chopsticks !) on the other for a whippier contrast. And then again, I've crunched the two kits together. There was one song my mate and I were doing and it had a reggae flavoured drum pattern for the 'verse parts' so it was quite sparse. We did the first tracking and we said yeah, we like this. But the drums had an ever so slightly thin sound so I asked him if he would overdub as identical a part as possible and he said yeah, OK. At the time, I had this Zoom multi-effects thing that allowed up to, if I remember correctly, six effects at a time. Anyway, while he was crashing away, sorry, drumming artfully (!) I was trying to get to the reverb. But I passed through the severe distortion and the sound of the drums through that was harsh and brutal. I said let's try it with that so we did. On it's own it was fizzing and I'm not sure that I would've used it. But overlaid on top of the original drum track, I liked that saturated drum sound. So we went with that. 15 years on, I still like it.
It's a can of worms you open up with two drum kits, but of the funnest kind. Worms get a bad press, anyway, especially from ladies.
Recently, having gone back to an acoustic kit, sometimes I get a friend to overdub certain passages that another drummer has done and I'll take out one of the kicks (or snares or whatever) or combine the two if possible or use bits of one and bits of the other, depending on which sounds greatest at that moment, but that's not really what you're talking about.
As a complete aside, I like having different drummers drumming on different parts of a song, for not only drumming contrast, but also for miking contrast. But these are just silly man's games with his toys - it's the endgame that counts.
There have been a number of artists (Adam Ant, the Outlaws, Miles Davies on 'Bitches brew', John Coltrane, even the Beatles tried it) that have utilized the two drum approach but I've rarely heard it work with them because it often sounds like just one kit which rather defeats the object in my opinion. Which surprizes me because I know it can work.
Have a groover, man !
