I am 6 months into the trombone - and on a 4 yr comeback on the trumpet. My take (so far) is that the trombone is taking more air, but when I switch to the trumpet in practice it really helps to "loosen" up the lips, or provide more "air" or something like that for the trumpet ---- and it is then, "easier" to bust out the high range on the trumpet (as long as it immediately follows the bone) --
so the trombone for me -- is helping my trumpet playing -- IMHO. (the trumpet playing makes the higher range on the bone - pretty easy) -- so maybe they help one another.
and FINALLY --- I noticed my range on the trumpet (decreased a couple of notes --- down from a somewhat reproducible high A to more like the high F ---- BUT the NOTES from the high C to the High F ---- are MUCH FULLER, and MORE RICH IN TONE and SOUND QUALITY, than they were Prior to the trombone doubling ------so, air, composure, mental aptitude, less tension --- WHATEVER IT IS ----- the doubling is HELPING on both instruments
and in general this is a "switching back and forth" in practice between the two instruments -- with about an hour on each total --- so, YES 2 hour practices most every day