>>106368301He probably planned for both to die, which is why he was stalking Mufasa's progress to make sure it happened, the same way he was watching the Hyenas when they chased Simba in the Elephant Graveyard. Being able to thrown Mufasa off the cliff himself was just a lucky situation in Scar's eyes where he got to taunt before finishing him off.
As for why he didn't kill Simba himself: It's often a classic villain thing to not do the dirty deeds yourself and to send your underlings out. Especially considering Scar admits a few times he's not one for brute strength. I mean, if you wanted to kill your nephew, would you strangle him yourself or would you get your henchguy standing behind you to do it? The only reason he fought Simba 1-on-1 in the end is because he was alone and cornered.
