Yay, Splice onto Sorcery! I've wanted to see that mechanic for ages; don't think I've seen anyone make a custom cardset with it though. So I'm glad to see you do it :)

Splicing this onto an overloaded Mizzium Mortars does weird things. "Tap or untap up to two each creatures."

Hmm. Boo. True. We might need to tweak the rules for splice to say that self-text-modification effects (i.e. overload) don't affect added rules text.

LOL, I'd not even thought of that. It's a shame they can't interact. Magic is most interesting when you get different effects that work sufficiently consistently you can discover interesting interactions between them.

You might be able to reword the comp rules for overload so it does something like replace "two targets" or "up to two targets" with "each". Although you'd have to write splice cards so they work with that (spells like "tap target creature and untap target creature" would be weird).

In fact, I think this is overload's fault not splice's fault. There's a reason there's no "all spells you cast have overload" card. I trust wizards there was no better way of doing overload spells, but it seems an ugly hack. IIRC, some foreign languages had a problem with the "replace...target...each" wording and spelled out what each individual card did in its rules text. That would at least work consistently with splice. So any fix to the comp rules would have to work with both.

I'd rather overload was written something like "if you would choose a target for this spell, you may choose any number of valid targets instead". Although that wouldn't get round hexproof, unfortunately. But since it wasn't, assuming wizards don't want to make functional errata, probably the only fix is to errata the english cards to match the foreign versions and remove the "replace target each" concept entirely.

However, I think it's fine to design splice cards that don't work with overload -- I think wizards must accept overload isn't as extensible as it looks, and may just say "we'll fix the existing overload cards somehow and then pretend it never happened". Which is a shame, but presumably if there were a more consistent way of writing overload, they would have used it the first time.

I suppose an alternative would be to make sure each splice-onto-sorcery card is overload-compatible. Make this say "You may tap target creature and you may untap target creature", or "Tap target creature you don't control. Untap target creature you control."

True, good point.

My own idea for fixing overload is to have it (and all other text changing effects) to affect AST rather than English text. (This also allows it to correctly affect things that say "any target".) However, there are still a few cases where this might not work, but it works better than dealing in English text. (I do think that it would effectively change this card to "tap or untap each creature", or alternatively, to do it twice but the second one is redundant.) However, as a further consequence of this change, another change being made is that implicit token names (due to the token's subtypes) no longer match names they have the same spelling as (although explicit token names still do match cards whose names are spelled in the same way). (You can discuss this and my other ideas about rules on my NNTP. See my "Ziveruskex and Strixan" card set (mirrored on Magic Multiverse, but designed using TeXnicard) for some of my other ideas about rules.)