Just a couple things I think are worth noting:
For the Spotter:
2. The hand-claw bipod technique. If you don't have a bipod-or anything for that matter-to rest your rifle on, use what we call the hand-claw. The easiest way I explain this is through a picture. So I made one on Windows paint for ya.
Don't do this, use a sand sock. It's cheap and simple: you get a sock, fill it with sand, and tie a knot in it. Instant bipod. Using it takes very little practice, you quickly figure out that squeezing it slightly brings the muzzle up, relaxing your grip brings the muzzle down, etc. Time tested and low-tech.
A vest that is MOLLE adaptable
On that vest and admin pouch (for paper and pens and stuff)
A smallish primary-M4, Mp5, G36, the like
Binoculars that you can see out of through eye protection
Papers and pens
That's about it.
You forgot water for both of you (so the sniper can crawl easier if necessary), and ammo, ammo, ammo!
A good secondary with plenty of ammo
The sniper and spotter should be using the same magazines on their secondaries if possible, for quick interchangeability.
Binoculars with higher power than your scope
A camo'd kind of blanket to set everything out on and pick it all up in a hurry
A very flexible vest, because you are going to be doing most of the moving of the upper body
Not a vest, either a simple belt or an ALICE-type Y or H rig. Something less constrictive than a vest. No crossdraw or drop leg holsters either, put your gun on your hip where it's out of the way of you're crawling through bushes on your belly.
I find the spotter, in airsoft, to be more of a "watch my back, man the radio, and help me make strategic moves that might require more than one person" kind of role than an actual spotter. They only time I've ever used a spotter as an actual spotter was once, when I was cornered and had no escape route, and wanted to take out as many as possible before they got me. The spotter was calling my shots in order of their immediate danger to me, and I picked off 6 or 7 of them before they were inside my MED. Otherwise, they watch my 6, they translate what I'm saying over the radio, and they help me distract, flank, leapfrog, etc. when moving or engaging directly.