This is correct.
If your opponent casts Swords to Plowshares on your Marit Lage you can respond by sacrificing a land to Sylvan Safekeeper to give it shroud, and if they respond to the SS activation by casting a second StP on ML you can respond by activating SS a second time. Once a single SS activation resolves, though, you don't need to activate it again until the following turn.

