If one targeted creature becomes an illegal target, the other will still get -13/-13 until end of turn.

You can’t target one creature twice to give it -26/-26.

Discarding a card and sacrificing a creature are both part of Ruthless Disposal’s cost. You can’t discard or sacrifice more to target more creatures, and you can’t cast Ruthless Disposal at all if you don’t have any other cards in hand or if you control no creatures.

You must target exactly two creatures to cast Ruthless Disposal. However, one of the targets can be the creature you intend to sacrifice to pay Ruthless Disposal’s cost.

There’s no way to discard a creature card with madness, cast it, and sacrifice it to pay for both of Ruthless Disposal’s additional costs.

Once you begin to cast Ruthless Disposal, no player may take actions until you’re done. Notably, opponents can’t try to remove the creature you wish to sacrifice.