I can't find the paper where it was discussed, but I remember reading through where someone tested four different methods for introducing/pairing broodstock. 3 of the 4 were pretty effective for getting them to pair while the fourth had frequent aggression.
I only remember two methods off-hand: one method was just dropping them into the same tank; the other was putting them in breeder boxes (or breeder box like setups) where they could see each other and sense the chemicals in the water from each other but not actually interact with each other physically. Dropping them in is the one that often resulted in aggression, so I wouldn't recommend that. I'd introduce them through the breeder box method. In theory, you should be able to tell if they're aggressive/calm and if they're bonded or not within a few days/weeks. (That said, these methods may be more effective on fish that do more swimming and less sitting in a burrow; I'm not sure, but I'd be curious to find out.)
With the shrimp, you can theoretically sex them (though you may need some magnification help - see the quote below) and introduce them to hopefully pair the same way as the fish; but if you're unable to sex them, then you'd basically just get two and skip to the introductions.
How to sex the shrimp according to Bob Fenner (I know this method holds true for Synalpheus spp., but I can't find any verification that it works for Alpheus spp. as well):
"Very hard to distinguish sexes externally... Females are characterized by gonopores with U-shaped slits on the coxae of the third pereopods (first walking legs) while males have oval gonopore openings on the coxae of the fifth pereopods (third or last walking legs"*
*Source:
http://www.wetwebmedia.com/alphreprof.htm