Well, I didn't really give you anything. Since no one else has either, I'll give it a try. Like I said, I usually am the only horn in my band (we've had an occasional guest horn to join me, but it's rare), so take this for what it's worth.
For this discussion, riff
means a repeated phrase throughout the progression, usually the same all the way through, but in some cases a note might be changed going to the IV & V chords in a blues, for example. Often a riff is based on the blues scale. A line
might be a head arrangement or a phrase based on the chord changes. I use the term horn line
in a more generic sense than riff
. A horn line could be almost any sort of phrase played as a background or as a featured part of the tune ("signature lick").
Anyway, riffs and horn lines can be played in unison, or an octave apart. So, for example, the trumpet (or alto sax) could play an octave above the tenor sax, but be playing exactly the same line. This can be very effective, whether you play in the same octave or not, and is fairly commonly done. It is obviously the simplest route and in many cases the best way to go when playing riffs "on the fly" with 2 or 3 horns.
If you want to harmonize, you need to pay close attention to the chords and use "voice leading" for each horn (or "voice"). In a nutshell, this means when moving from one chord to the next, you move to the closest chord tone, usually a half step or a whole step away, or even staying on the same note. This principle applies even when you're the only horn. So, moving from C7 to F7 (I7 to IV7 in a blues), if you're playing the 3rd on C7, an E, you'll move a half step down to Eb (the b7th of F7). Of course, you could move a half step up to F, but the 3rd and 7th define the chord, so it's more effective to use those tones.
Back to harmonizing, with a second horn you can play in 3rds (or 6ths, depending on which voice is on top). The 3rds will sometimes be major and sometimes minor, depending on the chord. Using the same example in C blues:
C7: horn one plays C, horn two plays E
F7: horn one stays on C, horn two moves to Eb
G7: horn one moves to D, horn two moves to F
A third horn could play a "tritone" away from horn two, covering another chord tone for each chord:
C7: play Bb
F7: play A
G7: play B
Or two horns could play the tritone (the parts for horn two & three above). This gives an edgier sound (with only 2 horns), but really brings out the dominant chord sound.
You can also play in a parallel harmony, where each voice remains a given interval away, usually a minor or major third. The interval may change from major to minor depending on the chord. A good example is the Jimmy Smith tune "Back at the Chicken Shack." Listen to the head and if there are two horns, one will be playing off the major third of each chord and the other off the 5th of each chord.
You can work this all out on a keyboard.
Hopefully this isn't too confusing. I just wanted to give you an idea of how it works. It's very difficult to explain clearly in this format, and hopefully if I made any errors someone will correct them. Or add something to help clarify.