If all your eggs are set the same day, a second 'bator won't be much added benefit other than insurance if the other goes down. (or, in my case, not insurance because I would have it full of more eggs!) A second bator is ideal if you want to stagger your hatch.
I run my GQF as an incubator and may have up to 6 different set/hatch dates going at any given time. The different sets are usually at least 5 days apart, but now and then something crazy happens and I'll have some just a day apart. Since the humidity requirements change on day 18, I also run an old medical incubator I converted to a hatcher. On day 18, eggs from GQF to hatcher and lock down with bumped humidity and no turner. (If I have eggs just a day apart, I'll fudge the later eggs by a day and move them on 17 so lock down isn't violated.)
I like this 2 stage hatch for flexible start dates, ease of condition control, and for the total lack of mess in my GQF cabinet bator. Definitely worth a second bator for my crazy hatch schedule.
All my incubators currently have eggs in them. I don't do staggered hatches, so "everybody" hatches in the incubator where they were first set. I don't want to mess with staggered hatches because I am old and forgetful. In my case, a second incubator was the opportunity to hatch more eggs set at a different time that the first one. Or the second one. Or the third one.... you get the point.
IF you do staggered hatches, it's a great idea. IF you think you won't start filling up the second 'bator instead of using it just for the lockdown/hatching phase, ... welll.. eh.