If Scheming Symmetry (controlled by an opponent of yours) was already cast targeting you and another player and Veil of Summer (you control) resolves—
you gain hexproof from blue and from black and so generally become an illegal target of that spell, but it will still get to resolve if the other player is still a legal target (C.R. 608.2b, 109.5; see also C.R. 702.11c-d), but
another Scheming Symmetry can't be cast by an opponent of yours this turn if that spell can't target two players other than you (including if that opponent is the only player still in the game besides you) (C.R. 115.1a, 601.2, 601.2c).
Spells only fizzle when all of their targets are illegal. Spells with multiple targets will try to do as much as they can if only some of their targets become illegal prior to resolution.
Would this mean that if I control a Leyline of Sanctity that my opponent couldn’t cast Scheming Symmetry in the first place, because there are not two different legal targets? Thanks.
Would this mean that if I control a Leyline of Sanctity that my opponent couldn’t cast Scheming Symmetry in the first place, because there are not two different legal targets? Thanks.
If you have hexproof due to Leyline of Sanctity—
you can't be the target of a Scheming Symmetry your opponent controls (C.R. 702.11c), and
your opponent can't cast Scheming Symmetry if it can't target two players other than you (including if your opponent is the only player still in the game besides you) (C.R. 115.1a, 601.2, 601.2c, 702.11c; see also my comment 2).
For example, even if you have hexproof, an opponent of yours might still be able to cast Scheming Symmetry targeting—
a teammate of yours and another opponent of yours (e.g., in a Two-Headed Giant game [C.R. 810.1]), or
two opponents of yours (e.g., in a Free-for-All game with three or more players in it [C.R. 806.1]).
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EDIT (Oct. 10): Correctness edit.
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For example, even if you have hexproof, an opponent of yours might still be able to cast Scheming Symmetry targeting—