First, for safety reasons, ATP does not operate single engine aircraft at
night, unless required to meet experience requirements for a particular
rating. This rule does not apply to the multi engine aircraft.
It is best to stick with the same instructor for the majority of your
training since they are the ones that need to sign you off for the check
ride. However, it doesn’t hurt to fly with another instructor from time to
time. A mutual agreement between all three parties needs to be made
beforehand because a lot of things have to be considered.
If I was your CFI, my first thought would be, “Would it be beneficial for
you to fly the next lesson without me?” Sometimes the answer is “No.” There
are some phases of the program that have a very limited amount of flight
time available to get you check ride ready. I may choose to not give any of
those lessons up.
On the contrary, other phases like Private and Instrument have a lot of
repeat or building block style lessons. Those are the kinds of lessons that
I would use to have my student go up with another instructor. Also, I would
be selective of which instructor that would be. Each instructor has
something different to bring to the table. I would prefer to put my student
up with an instructor that I felt was best fit for the job.
To summarize, it depends. Sometimes it’s a win-win situation. Other times
it is not. The three things I like to ask myself in these kinds of
situations is, “Is it legal? Is it safe? Does it make sense?” If the answer
to any one of those three questions is “No,” then I would make what is
called a “no-go decision.”