what would the two compressors be doing?
It’s a technique for smoothing things and also making them more exciting, someone recommended it to me a long while back, but it’s not a hard and fast rule. The basics are that you compress first very hard with say 12dB of reduction with fast attack to tame the peaks quickly, almost limiting in a way. This pretty much flattens everything out. Then to add some bounce back in, you compress 3-4dB after that with a slow attack and fast release. The slow attack then increases the dynamics by allowing some of the peaks back in before compression occurs, hence you get a smoother vocal overall but with some excitement included. That’s the theory I believe anyway.
Using a multiband compressor is also a useful modern day tool, but you would probably use that when there is a problem frequency which occurs occasionally during the performance. And maybe in this case it’d be better to automate an EQ change during the song, which makes it much easier to control.
I would say that it is now standard practice to automate much more to do the job (or even the artist!), simply because we have easy automation available. If you read some of the SoundOnSound articles about mixing famous songs you will see that they’re riding vocal levels throughout, to emphasise the words and the feel. It’s not just ‘set and forget’ mixing using just inserts anymore.
Finally, if you split the tracks (probably mentioned above) then you simply need to send both to the same group and you’re back with one track again. I do this sometimes for wildly different sounding takes - like AM/PM. So I’d split out, EQ one to sound like the other, then buss to a group where all the inserts are applied. It’s a pain but that’s the way it has to be to correct that sort of problem.
But there are no hard and fast rules Hope this is useful.