In the Librem 5 General development report they state “Since the modem used on the actual phone may be different from that of the dev kits, some initial testing has begun on the Gemalto PLS8 modems.”
https://puri.sm/posts/librem5-progress-report-21/
https://www.gemalto.com/m2m/solutions/modules-terminals/industrial-plus/pls8
I have seen lots of questions asked about dual sim but haven’t seen a definitive answer, I think they might be trying to find space in the phone but because they are using custom components it might not be doable space-wise and has a low priority?