If you are shooting with 2 cameras, forget the flash, too bulky and clumsy, at least for me.
If you need the flash, my preference is to shoot with one camera; get a 2nd shooter for the other camera, or ditch the other camera.
My preference is to shoot with 2 photographers. Plan and coordinate where to shoot from, and there is less running around.
Lens selection really depends on where you have access to shoot from, or are stuck in. And how much moving around can you do, and want to do, during the graduation?
Just for reference. In the film days, years ago I shot high school graduation with one camera, a 43-86 and 24 lenses, and a Honeywell Strobonar 800 flash. I did not bring the long lens, as I was already carrying borderline too much gear. The 24 was for the family gathering after the ceremony, as everyone is packed in very close, so you can't backup up very much.
First question, why do you need a long lens in a gym? Maybe if you are trying to do a TIGHT shot of just that grad in a mass of students. Or a long tight shot of the speakers? If you don't need it, ditch it. KISS
1st option
Try to rent a D850 (preferably), 810 or 800. Then you are working all FX and all D8xx cameras.
24-70 f/2.8 and 70-300
2nd option, similar to astro's recommendation.
FX - D850 with 24-70 f/2.8
DX - D7100 with 70-300 (105-450 FX equiv)
3rd option, one lens kit.
Rent a 24-120 f/4.
FX - D850 with 24-120 f/4 (this eliminates lens/camera changing)
(DX - D7100 with 18-105, as your backup camera.)
4th option, one lens kit
DX- D7100 with 18-105. This is a 28-158mm FX equivalent. The 18-105 is slow, so you have to crank up the ISO or use a flash.
If you want more reach, rent the 18-140 (28-210 FX equivalent). The 18-140 is slow.