I was just practicing and my train of thought went from making a rhythm track for myself of St. Thomas to how trading 4's and trading 8's works between two melodic instruments. I understand how it works with drummers -- the entire form keeps moving. But if, say, the sax player and the trumpeter in a quintet were to trade 4's while keeping in the vein of playing-what-the-other-plays-with-a-tag (eg, what I almost always hear when people do this,) how does this affect the form of the song? Do you just pick a section of the tune -- say, the bridge when trading 8's on a standard -- and repeat those until the two are done? What's the deal? Sure, there's common notes across chords, but I hear some guys trading back and forth for ages and I've never seen a non-modal tune where keeping the structure moving as normal with that is even remotely feasible.
People pretty much always trade over the form of the tune. I've been on some gigs where everyone got so screwed up that the rhythm section dropped out and waited for a cue to come back in, but ordinarily if two horn players are trading (or any two instruments for that matter) then they will just trade for about 20-30 choruses of the tune.
Check out some of the first Miles quintet recordings. On a lot of that stuff you'll hear them improvising on a tag vamp at the end of their chorus and then when you break for the next chorus you're at the top of the form again. But I don't get what you mean when you say "...in the vein of playing-what-the-other-plays-with-a-tag (eg, what I almost always hear when people do this,)...". Generally, even when trading 4s/8s, the form of the tune keeps going forward and the challenge is to continue the musical thought over the harmony that you're playing over. With a Quartet (tenor, piano, bass & drums for example), you have the following options: 1. Tenor, drums, tenor, drums; ad infinitum (and have the option of continuing with the other instruments in the band replacing the tenor position) 2. Tenor, drums, piano, drums, tenor, drums, piano, drums; ad infinitum (and having the option of adding the bass and going T/D/P/D/B/D ad infinitum) 3. Tenor, piano, (possibly bass), drums, T/P/pB/D; ad infinitum 4. Tenor, piano, tenor, piano, tenor, piano (or piano, bass etc. or whatever) But WHATEVER is happening, the form generally continues.
There is great cut on an old Mingus records, East Coasting (wonderful early Mingus record, originally released on Bethlehem, now available on CD) where three horns trade fours for a chorus or two, then trade twos, then ones, and so on, and end up trading two beat lines around and around. Great fun. Drummer is not involved in the trading. I have forgotten the name of the tune, I am not at home now to check. The whole album is full of great writing plus great improvising. If I recall the pianist is a young bill evans.