Heres some advice....and I know what I'm talking about.
Dont use 5 minute epoxy, opt for the "standard" epoxy insted. (its thinner)
Mix the standard epoxy 50/50 (resin/hardener) and then warm it with a blow dryer. (this thins the mixture) Apply to both surfaces, wait 5-7 minutes, then apply more (antlers are porus). Mate the two pieces together immediately, and position is such a way that they don't need to be clamped to hold together. Wipe off any squeeeze-out immediately with a cotton rag dampened (just damp, not dripping) with acetone. Wait 24 hours, then it will be good as new. In fact, the joint wll be stronger than the surrounding antler.
Cheers,
Ryan