If I build a pair of bows....

each would be 68-72" bamboo backed hickory, 35@28, almost center cut  (open to thoughts on length)

one guy draws 28" the other 32"

hoping to build each a half dozen arrows to go with the bow.

I'm open to spine suggestions, as well as field tip weight. Both bows will be for target shooting.

Length of the bow has little effect on spine. What kind of string material are you going to use?
Will the bow with the longer draw be 35@28? Or 35@32? Biiig difference there. Are the bows straight limbed or R/D? How close to center cut?
I'm ready to suggest spines; but need to know the answers first.

Both bows will be 35 @ 28.

Straight limbed.

A hair less than center cut.

I deal with 26" draw or less usually but, I'm willing to learn...Thanks for any input.

Assuming the strings are dacron and that window is really going to be cut that close to center, bow one (28") will need arrows 35-40# with 125 grain points and cut to 29" BOP.
Bow 2 will be producing 45-48# at that draw and with Hickory I would make it as long as practical-70".
POC will be tough to get at 33". Maybe Fir from Surewood. I'd go with 60-65# for that long a shaft and a 45-48# pull.
Add 5# for a "modern string material" like D97.
You might double check with the guys in the Bowyer's Bench section.

Thanks Bjorn!
I'll check the Surewood stuff.
I think they will both require shorter shafts, they are measuring their draw for the machines they shoot now. But, I have a starting point.
Yes, on the dacron and center cut.

You're right, Billy.  If they're shooting machines now, they'll drop down at least an inch, maybe two when they switch to a straight gripped longbow.