At the moment, no manufacturer actually offers 2x 4g modems, max 1x 4g and 1x3g. When the 2g network is effectively switched off in 2020, most mobile phones will be obsolete in this respect...there will be no update. It is conceivable that the upgrade would only be possible via hardware.
I have already used this phone in Thailand and the UK with two SIM cards, one for the country and one from Salt in Switzerland - both work at the same time. Only two constraints:
you have to remove the micro-SD card if you use two SIM cards - one of the slots is for one or the other
only one SIM card can connect to 4G at a time, the other can only connect to H+. But since it would be roaming to mobile data in the case of the Swiss card, it never caused me any problems. Apart from the fact that you have to remember to switch back to 4G for the Swiss card when you return to Switzerland, otherwise you wonder why the connection is so slow