Just a thought, but if lever is not shifting the pistons:
1. Are you sure they are not seized?
2. Have you tried cracking a bleed nipple open or opening the master cylinder (depending on type)?
Unless there is equal volume available in the system (ie, master cylinder level is lowish or can overflow/vent) then the fluid you are displacing by trying to spread the pistons has nowhere to go (unless a seal gives up…)
Obviously you may need to bleed after this depending on feel (you may want to anyway)
For tools, Avid do a little plastic wedge. A small wooden wedge should also do. Personally I use whatever lever I see fit between the old pads I’m planning to bin…