No, we don't frown on that. I've been in both ends of that - the primary and substitute CFI - a number of times. The biggest issue isn't ego. It's varying techniques which can be confusing, so it's best if it's coordinated between the instructors. But it's not always necessary - more about the student than the instructors.
A month or two ago. I subbed for a vacationing CFI with a presolo student. I asked the primary what the student needs to work on and that's what we did. Student soloed shortly after the primary's return.
Some years ago, my presolo student was on vacation and decided to take some lessons. We did not coordinate but the student was both smart and proactive. The other CFI offered to solo him. My student didn't go for it. When he returned, I told him he should have taken the opportunity.
sometimes it's not even about scheduling, I have referred students to other instructors when they were having a problem I couldn't figure out. And vice versa.