Are you talking about SIG 550 type magazines
If it's a SIG 556, it uses STANAG (AR-15) type mags. Magpul Pmags are the best AR-15 mags out there in my opinion. They are cheap and they work. Don't bother with the SIG polymer mags. They are junk.
Couplers are available to clip Pmags together, but in my opinion, this is a novelty that adds weight and unbalances the rifle.