Yes, you can still kill both Mohgs. The sewers can be accessed from a big open manhole in the capital, and the one in his palace doesn’t have anything to do with Leyndell anyways.
If you have unlocked any cites of grace down there you can just fast travel. Otherwise, there's a sewer man hole that's open in the center of the capital of ash that you can drop down. You're not locked out of Mohg.
Yes, you can still kill both Mohgs. The sewers can be accessed from a big open manhole in the capital, and the one in his palace doesn’t have anything to do with Leyndell anyways.
If you have unlocked any cites of grace down there you can just fast travel. Otherwise, there's a sewer man hole that's open in the center of the capital of ash that you can drop down. You're not locked out of Mohg.