Depends on how you hunt and what you're after. We always bring our own, just in case we end up splitting up to chase sheep in different drainages. If you're hunting on the "anything" legal route, having 2 spotters is very handy so two guys can be watching closely for that clear shot of legality. If you're trophy hunting a big ram only, then one would probably suffice, since it's just a matter of how awesome of a ram it is and whether you want to take that ram, or keep combing the mountains for a better quality ram. Some guys go as uber light as possible, down to sharing a rifle, so pick your poison.
Have always shared the spotter on a sheep hunt. Usually a 65mm of some sort. Both parties carry bino's as those get used 10x as much.
Pro tip. When splitting up the gear, volunteer to take the tent and let the other guy carry the spotter. Once in the mountains, the tent often stays in camp, but the spotter gets carried everywhere!
I've always considered the rifle sharing route too. That would drop more weight than anything, but haven't done it yet. In the rare instance of multiple FC rams in a bunch, it has been my experience that if you shoot the leader the rest will stick around and changing shooters would be a non-issue.
Two bino's one spotter for a two man hunt is how we've been doing it. If a buddy wanted to bring a spotter along I wouldn't argue but I will always have one with me as I love to glass.
Not the gist of your post here, but I'd be leaving 15's at home. Hell, I'd leave the 10X50's as well. Little to no advantage to lugging big bincs with a spotter along.....
Smaller the better... preferably 8 or 10X32s for each and one spotter.
Regarding gun sharing, a light, accurate rifle is a great help. For example, a NULA can weigh 6 lb. ready to hunt, and by the time you add accessories, most stuff is 8.5-9.5 lb.
I’d cut other gear before I dropped optics - gives options of splitting up etc. I might change my mind if chasing Dall sheep though - bighorns can disappear in little drainages it seems like
Rams tend to appear and disappear in seconds when glassing large, vast areas. The more eyes out searching....the better! I always enjoy watching and field judging rams so there's no way I'd leave the truck without my spotter. I'm constantly shifting back and forth from binos to spotter.
With that said, there's obviously a big difference between bivy hunting..... lugging a heavy pack around all day with your camp on your back.. and hunting from a spike camp with a lighter load. I find myself packing the basics when bivy hunting but am more willing to haul more stuff when hunting close to camp....especially in "reasonable" terrain. If I'm going on a death march I may cut back on items.
Sometimes I leave my tripod at the truck/camp if I'm only glassing a small area for a short time. I'll place my spotter between rocks on my backpack. I often bring along a Canon camera that takes pics/video with a 35x lens. It's possible to zoom it in and get great footage but would be tough using it for spotting game.
I'm constantly going through my gear and figuring out strategies to loose weight and bulk on each particular hunt. There was plenty of water where I was sheep hunting this year so I bought a new "Be-free" . If you think about it, water is one of the heaviest items/volume in your pack. With the Be-Free I didn't have to lug around water all day and drank from the many water sources with the Be-free as needed.