On the Stack 9: An idea for an Mechanic

One night last week I dreamed I was building a Dominaria sealed deck, but the cards were all different, I kept finding rares I couldn't use, and every time I looked through the deck I was in different colors. In the middle of all this was a mix-and-match mechanic, like a cross between the split cards with Fuse from Dragon's Maze and Unstable's Host/Augment mechanic. I was still thinking about that mechanic when I woke up, and I started trying to refine it so that it might work, if not in tournament-legal Magic, then at least in silver border.

Let's call it Synchronize, and make it a keyword ability on instants and sorceries. Spells with Synchronize can be cast like normal, or any two can be cast as if they were one spell, for a mana cost slightly lower than the sum of the two spells' costs.

Here's a rough draft of an example.

Time to Build Up
1G
Sorcery
Put a +1/+1 counter on up to two target creatures you control.
Synchronize (You may cast this card at the same time as another card with Synchronize. If you do, pay 1 less than the total casting cost of both spells, and the two spells resolve and can be countered as if they were one spell.)

Time to Break Down
1G
Instant
Target creature you control fights target creature you don't control. (Each creature deals damage equal to its power to the other.)
Synchronize (You may cast this card at the same time as another card with Synchronize. If you do, pay 1 less than the total casting cost of both spells, and the two spells resolve and can be countered as if they were one spell.)

So you can cast Time to Build Up for two and a green, or Time to Break Down for one and a green, or cast them both together for a total of 1GG. If you synchronize two spells with different colors in their mana costs, you pay all the colored mana; the discount comes out of the generic part of the cost. For example:

Time to Stop
1UU
Instant
Counter target spell.
Synchronize (You may cast this card at the same time as another card with Synchronize. If you do, pay 1 less than the total casting cost of both spells, and the two spells resolve and can be countered as if they were one spell.)

To synchronize Time to Build Up and Time to Stop, you would pay one generic mana, one green, and two blue.

The remaining big question is about the card types. Sorceries can only be cast on the caster's main phase; instants can be cast at basically any time, including during the caster's opponent's turn. It makes sense for two synchronized sorceries to be cast as a sorcery, and for two synchronized instants to be cast as an instant. But if you synchronize an instant with a sorcery, do you increase the combined spell's power by allowing it to be played as an instant, or decrease it by making it a sorcery? It depends on what the cards with Synchronize end up doing, and how powerful they need to be to fit into the set.