Any suggestions on how to narrow down which latex mattress products would have such edges? We have decided to go with the customizable route (i.e. will have zippered encasements). It’s hard to tell from the vendors’ websites which might work best at minimizing the gap.
I can’t give advice on how to ensure a minimal gap, but I will say that having bought two side-by-side Brooklyn bedding latex hybrid twin xls (no longer offered, I believe), we found the gap to be very noticeable and ended up having to buy a foam wedge to bridge it. That’s worked well—though it bulges slightly—but could be an option to put inside the mattress casing if you find you notice the split. The mattresses we got have pretty firm edges compared to some pure latex mattresses I tried, so I think the risk might exist with any brand.