Sylvan safekeeper shroud not working against shatterskull smashing split between 2 targets

If an opponent targets 2 targets with shatterskull smashing, and I sack 2 lands to give each creature shroud with shatterskull smashing on the stack, it still resolves without fizzling ( even though both targets are illegal) and i'm down 2 lands and 2 creatures cause it ignores shroud and they die anyway.

Start by reproducing the interaction between Sylvan Safekeeper and Shatterskull Smashing when the spell targets two creatures and both targets gain shroud while it is on the stack. Trace target legality and resolution for this split-target case; done means both illegal targets are handled correctly and the protected creatures and sacrificed lands are not incorrectly lost.

Written by the indexing model from the issue text.