ok so I have Lydia. I have her "wait here" at my pad.
I go to Riverwood and talk to Sven (who I hate) to follow me, so I can sacrifice him. but he keeps saying I already have someone....
???

do I need to "part ways" with Lydia, or is there someone else I need to talk to to get them to follow me so I can kill them at the altar???
are there other NPC's I can accomplish this with?

thanks

This. "Wait here" doesn't dismiss them.

Yeah, you have to select "it's time to part ways" to completely dismiss them.

Don't worry, she won't go anywhere. You can go back to to your home and recruit her again.

Yep. Part Ways.

I remember seeing that optioin the first time I ran into a situation that called for me to dismiss Lydia. I was afraid it meant I was firing her: it does not.

I've gotten into a loop of relaoding and backtracking, not advanicng lately. I need to level up and do this quest, cant ignore my Daedric Lord.

Yeah, the "wait here" command lets your current follower remain active until you return to them. Generally for scoutng ahead so you dont get ambushed or so the follower wont trigger traps. Or drawing enemies back towards the follower.