I also did my FFS in 2 steps:
Jan 9, 2016 - rhinoplasty
Sep 10, 2016 - forehead, browlift, upper blepharoplasty, trachea shave
The rhinoplasty was completely necessary. I had a huge, very masculine-looking nose. Also a big bump on the bridge of my nose. Dr. Zukowski did a perfect job making the dramatic change necessary to make my nose look very feminine. The rest of my FFS was maybe more optional, since I had no problems passing after my rhinoplasty. I still felt it was needed for my own well-being though, so I had those additional procedures done. These changes were much more subtle I think, but also very effective. Again I went with Dr. Zukowski for the second time.
I agree with archlord that it is better to do all the FFS at once. First of all, it will cost less money (since you only have to pay once for the anesthesia and hospital/facility costs). Post-op recovery also really sucks, so it's nice to only have to go through that once (although it could be argued that if you split it up into multiple procedures, you may experience less pain/discomfort each time, which might make things easier overall).
But to your main point, since the nose and forehead are so close in proximity to each other, adjusting one will affect the other. That is the primary reason to get them done both at once, so that the final effect looks more natural. And I think you are right to be concerned about having rhinoplasty done twice, as I believe that the chance of a good result diminishes each time (although I have heard first-hand accounts of really good rhinoplasty revisions too).
Ultimately, the decision is up to you. If you want to go about it in multiple steps, you can certainly do so. Archlord appears to have had a good result doing it this way, and I most definitely have as well.