One of the challenges of playing in multiple combos is being able to blend well in each. Even in one group, it can vary: it’s common for sax players to switch between alto and tenor and flute, for example, and the horn section in Play It With Moxie! changes lineup on every song.
Also, although we’re usually playing standards and often the same arrangements, each player learned it a little differently or takes different liberties with the rhythm. When you rehearse weekly, you have the luxury of getting familiar with the other person’s patterns. Otherwise, you probably won’t match exactly on unison or harmony parts (as in the quintet’s Comcast videos). You can finesse it a lot more easily on harmony than in unison.
Harmony is a lot more forgiving for intonation as well. Every instrument has its bad notes, and they’re not the same. I know I have to be careful of low D and C# and middle A and high F, etc., and I know a few of the evil notes on other instruments. Sure, you can say that a good player will play in tune, but that goes out the window when there’s an HVAC vent right above you.
My favorite part of blending in combos is mixing different tonal colors. When playing with a flute, I usually use a cup mute, specifically a wood one (makes sense, right?). There are some exceptions–the popular version of “Cape Verdean Blues” was flute and open trumpet, and “Lorraine” often is flute and harmon mute. Also, I stay in close harmony, though often in sixths. An Alto flute is great for close thirds and unisons; octaves with a regular flute is asking for trouble. I also like to invert, to have the trumpet above the flute on the occasional long note, but that’s hard from a volume standpoint. With a sax or trombone I’m most often open, which actually is a little more forgiving on intonation, partly because I don’t hold back as much (playing quiet and in tune is the kind of thing trumpet lessons drill on). Sometimes I’ll use a straight mute against a tenor sax, but it’s a more strident sound than some venues call for. Usually if the bone is using a mute, I’d use the same one, although mixing plunger and wah-wah is interesting.
Unison and harmony are great sounds, but a lot of the time we just have one person play the melody and the other do decorative licks during long notes.