Twin Souls changes the actor value that controls the max commandable actor amount. If you set up a spell that summons two minions, you automatically keep them if you own the perk. "Elemental Potency" is more or less just a condition perk. It does nothing on itself beside telling spells what to do.
Let's take the "Conjure Flame Atronach" spell. It contains two magic effects. The first summons the normal Atronach and the second effect summons a potent version of it. If you just have the "Elemental Potency" perk, the one from the first effect is immediately destroyed and you keep the potent version. With a present "Twin Soul" perk, you keep both Atronachs.
You can look at these records to get an idea how they work together:
- ConjureFlameAtronach (spell)
- SummonFlameAtronach (magic effect)
- PerkSummonFlameAtronachPotent (magic effect)
Basically you only need to create a duplicate of your currently used magic effect. Associate a stronger NPC with it and place the same conditions on your effects like the two effects mentioned above hold. After that add the new effect to your spell.