I'm going to spend October through February building a base by riding at least two hours in Zone 2. I intend to do this four times a week. Is there any drawback to taking a break half way through the two hour session?
Your post doesn't indicate what "zone 2" means to you in terms of perceived exertion. If "zone 2" is supposed to be easy, then stopping a few minutes make no difference to any fitness benefits. If you are stopping because of discomfort or fatigue then you need to address the specific bio-feedback and correct your exercise session's metrics. (use shorter periods, change bike fit)
Typically, the
largest percentage of aerobic benefits is realized in about 50 minutes of activity. Unless your base-building work is
being conducted in unison with high intensity work, your extra-long sessions are worth even less.