You can combine whatever you want in this case. Nobody is stopping you. You will not be arrested by the "sound police" for the wrong combination!
I would probably benefit you most though to combine the two tracks that you don't feel you will need to change eq, or the combined mix of later on. In this case, you might consider the drums and bass. The folly of this is that if you don't get it right when you bounce the tracks, there is not way to "undo" the bounce if you recorded over those tracks already. So, you HAVE to get it right or live with the consequences. Sucks eh?
Experience in doing it wrong a bunch of times is usually the best teacher here. Before you do it with important material, you may want to experiment with stuff you don't care about, or just record something new using similar tones and practice before you do the bounce with the stuff you want to keep.
I remember when I got my first 4 track. Took about, oh, 1 hour to realize that I needed 8 tracks! When I got my first 8 track, took about an hour to realize I NEEDED 16 tracks! Got that second ADAT deck, and quickly realized that I could do with no less than 24 tracks!!! Recorded a CD for a funk band once, and we HAD to have 32 tracks!!!! But on one song, we sure wished we had 40 tracks!!!! We had to put three horn solo's, all at different times in the song thank god, on the same track. At mix time, we ran 3 compressors chained together on that track, and would set each for one of the solos, then put the other two in bypass mode while a certain solo was playing. The guy helping me mix would then switch the current compressor to bypass, and enable the next compressor for the next solo. On the console, I had to manually adjust the eq for each solo before it started. Fun eh? But that is just part of engineering.
Good luck.
Ed