Why just 2 horns?
I only wrote about 5 charts using 4 horns early on for the Denver Brass. In 1988, at my urging Denver Brass went from 4 (or sometimes 3) to 2 horns. Since we were relying heavily on the Philip Jones library for programming, it made no sense to have that many horn players sitting around. It did make sense to occasionally double up the horn part on some of the PJBE charts (see comment below). During my few years as Denver Brass Music Director and then Artistic Director, I wrote a lot of 423.02 charts. But there was always too much tuba in the mix (and I am a tuba player). It also meant that when playing PJBE charts, one tuba was playing the bass trombone part, and that simply wasn't the right sound. I love having the option of 3 or 4 different colors for the bass voice (tuba, bass trombone, euphonium, or a tenor trombone - not to mention a number of other possibilities by combining those voices both in unison and in octaves as the bass voice).
Why did PJBE only have one horn? There is a great book called Odyssey of the PJBE by Donna McDonald (Editions BIM) that chronicles this in more detail. Philip started his group as a tower music brass group of 2 trumpets and 3 trombones. A little later, on a trip to the US he heard New York Brass Quintet with a tuba (Harvey Phillips) and was hooked. He started a separate group of 211.01, using my personal tuba hero John Fletcher. He later found occasion to combine the forces of the 2 quintets and...414.01.
I always felt that the single horn voice got lost in all the directional instruments. PJBE had some fantastic horn players (Ifor James, Alan Civil, John Pigneguy, Frank Lloyd) that could certainly hold their own, but still...
PJBE was usually chamber music and that was our other big goal from the outset with Boulder Brass - a large group that could still play chamber music but could also pin your ears back. 4 horns makes the chamber music part a little more difficult, especially since great horn players are rare (comparatively speaking) and in high demand for other engagements. Plus you can do a lot with voicing 2 horns by writing notes on reinforcing partials (octaves, for example). Most early classical orchestral music is only 4 horns so you have enough note coverage on the natural horns - this is where high horn / low horn got started).
This is also why I like the euphonium in the group. It plays many roles - tenor tuba, 4th horn, solo tenor voice, 2nd or 3rd trombone sound, bass voice of a high conical choir, etc.
Most of my charts have a flugelhorn part - another tactic borrowed from brass band music. Flugel can be 1st horn, top voice on a trombone choir, rounder bottom voice of a trumpet choir, lovely alto solo voice, and so on.
I'm hooked on this instrumentation, it always makes sense to me as I'm writing, even for music with limited original voices (like a 4 voice Bach fugue) - that's when it really gets fun to play around with the orchestration color combinations in the group. It's also one more voice than a pianist has fingers. It's exactly the right number of voices for organ music (ten fingers and a pedal voice). I could go on...and probably will.