I just went through that when I ordered Moto G5+ for my wife and myself. Our coverage at home is so-so for both the GSM and CDMA carriers. I was more interested in the coverage across the nation as we travel, so I wanted one phone to be GSM and the other to be CDMA (Sprint). I bought one phone at Walmart, ensuring that Republic would send a GSM SIM card. I bought the other phone from the RW store. When shopping in the store (before checkout) I used a location that showed NO coverage for GSM, but did show coverage for CDMA. Half the phones were grey’d out. (The G5+ was not since it works on both networks.) I ordered it from RW and a few days later it arrived. I was happy to see the green CDMA sticker on the box and the SIM pre-installed. I don’t know if RW provided the CDMA because I was shopping in a CDMA area, or because they picked it from my home zip code which could have had either. In any case, RW shipped the phone I wanted.
We are now very happy that we have dual carriers when traveling together.