I ran dual sim card holder in which each sim card was cut down to size (from a provided template) and inserted into a carrier that was the same size as the original card. Switching between sims was a software selection, they couldn't both run at the same time. Also included in the pack was a blank to insert one of the cut down cards to run the phone off one card. The carrier was thin aluminium and very slightly thicker than the original card ( cigarette paper thicker.) That extra thickness, in my phone, led to deeper than design tolerances indentations in the sim contacts and eventual failure of one of the cards. It could have been my fault for repeatedly extracting the carrier to show it off!
Most if not all modern phones will have the software to run 2 chips.