Split cards are two cards on a normal sized cardstock. In reverse, we have Divided cards. These are split cards that are printed as full cards. They may come with a keyword called Join. When you have a join left and join right cards, you may play both of them at the same time. Put the join right card on the stack first, then the join left card on top of it.
Then we have Twin cards. Like Divided cards, which only work for instants and sorcery, Twin cards are full cards, but may be combined on the battlefield. Thus, Twin cards are permanents only. The combination cost is the total of Combine costs on both cards.
Can different types be combined?
All characteristics are combined: power, toughness, color, abilities, types, mana cost, etc.
Counts as one card when combined, i.e. for targeting, attacking, blocking, etc.
Two combined creatures can block an additional creature.
Creature - Merfolk Wizard
: Target creature must attack or block this turn.
Creature - Goblin Berserker
When Misty Merfolk and Gobbly Goblin are combined, they count as a single 4/4 creature with first strike and "
: Target must attack this turn", and can block up to two creatures.
Another ideas is like those 3d books. A panel divides the card and can be flipped back and forth to show the two halves.