I think the answer depends on why do you need 2 SIM's, some thoughts...
Assuming - at least in the UK - for none of the [consumer] carriers support eSIM...
2 Numbers - on at the same time - select a carrier that will let you have 2 numbers
2 Numbers - switchable, you might be able to use an adaptor as isuethis says. For travelling have one in a old Nokia phone that takes 2 nano sims and fits in a normal SIM sized works great (one UK sim one French SIM ). It's tight fit though and might not fit the Gemini Quality varies a lot though the 1.99 ones on ebay are probably not that reliable. Some use SIM toolkit menu other older ones have a tiny switch. you could always metal bash the Gemini a bit - or perhaps not
Low tec Solution for traveling - stick the second SIM inside the case and change when you get there...
Better network coverage - there are now a few MVNO's that roam networks within country e.g. Anywhere SIM
https://anywheresim.com/ or Pebble
https://pebblenetwork.ltd.uk/