I've done the "add a different sky" print before now - It is very much easier to set the negs up in separate enlargers and fine tune the masks with test prints. I had the masks set a couple of inches below the enlarger lens to avoid any hard joins.

Also had an issue with the halo effect when masking out an area for some dodge/burn/contrast trickery - A bit of time with some spotting ink reduced the halo to barely perceptible proportions.