Chest/Dark room and Mega Satan in one run?

[Lucathegreat]

I can't quite recall right now, but can you do both?
Like, kill ???/Lamb, then go back and fight Mega Satan too, so to get both achievements in one run?

[F153n]

Yes, as long as you do ???/Lamb before mega satan.. ( and dont enter the big chest before mega satan ofc:) )

[Frank]

Yes, you get the completion mark and the achievement as soon as ???/The Lamb is dead. There's no need to enter the big chest.
However, entering the big chest and killing Mega Satan give you different endings. If you already unlocked the ending then it's fine.

[Lucathegreat]

Thanks, yeah! Was having a good run with Lilith and managed to kill Lamb, the achievement popped up immediately and I could backtrack and enter the big door.
He killed me, though :(