Thanks for expert comments!
SIM switching is one thing, but, having each SIM locked to a different provider network or even antenna on a tower is another thing.
It is possible (if I read modem manuals right, but, very cryptic and a bit scary) to do so in AT (I found some hints and manuals online).
Regardless of amount of SIMs, can git OpenWRT lock a SIM to a particular provider network? I think (and I believe did with one SIM) so with 'option plmn' and I guess perhaps that will always choose particular provider antenna on a nearby tower. Or does tower locking work differently?