Most likely ypu have a restriction 'upstream' from the calipers, as your explanation proves. The pistons are free, one can move the other, but both cannot move in at the same time because the fluid cannot escape the caliper. Undo the brake hose to caliper connection and I will bet that both pistons can be retracted.
Rubber brake hoses can swell up inside, invisible from the outside, which can greatly restrict fluid flow and can cause brake pads to stay pressed against the disc, causing it to become red hot. Ask me how I know! But usually only after many many years.