Hi, I don't know about shared SP/Spheres, technically you can play a lvl 5 turn 1, also 9 slots for both players makes subtype based files hard to play.
This got me thinking, here is my idea of 2vs2:
The fields are 3 rows by 5 columns, for ab vs cd, the field for ab is
1a 2a 3ab 2b 1b
4a 5a 6ab 5b 4b
7a 8a 9ab 8b 7b
We have side_a=1a-2a-4a-5a-7a-8a , side_b=1b-2b-4b-5b-7b-8b and shared_ab=3ab-6ab-9ab, so field_a=side_a+shared_ab and field_b=side_b+shared_ab.
- player "a" can place units only in field_a (side_a + shared column);
- the SP and Sphere Levels are not shared;
- all damage abilities are limited to 1 field, so any damage (target or AOE) directed at field_a doesn't affect side_b (the shared area is affected);
- all buffs from side_a affect field_a, all buffs from side_b affect field_b, the buffs from shared_ab affect both field_a and field_b. So any unit in the shared area can receive(and give) buffs from both the fields (but also receive damage from c and d);
- all offensive abilities from side_a can normally only attack field_c (side_c+shared_cd), if there are no unit in field_c, "a" can iczer attack player c or attack the units in side_d, units in shared_ab can choose which field to attack, the same rules apply to soul and open skills, so Fierte SS will only work on the a and c players(I'm still thinking about how 1 time buffs will work on the partner)
- player a can switch positions with b so he can attack field_d using special abilities (lets call them gambits to avoid confusion, i'll talk about them later);
- units in shared_ab can be used by both players, the sp from the player that selects the ability is used, so player a plays samurai girl and b can consume his sp for slash, this is only valid for units in the shared area; if samurai girl dies player a takes the LP hit, this can be avoided by moving samurai girl into side_b (possible with normal move or a gambit);
- partners can play the same card without backlash, except characters;
Now lets talk about gambits: both groups have a set of shared abilities with different cool-downs. A single gambits is selected at the beginning of the turn, at the same time as the cards are played. The gambits activate before the open skills. Here are some examples:
- Trade Places: player a trades places with b, now a with his units fights d and b fights c, note that only the sides change (the field is just mirrored);
example: player a uses archer pixy, normally he can lock c's file, now with the switch he can lock d, but he must fight d's file until he can reuse the gambit again, of course c and d can foresee this move and also play Trade Places to cancel the effect
- Mirror field: if played by "a" the columns 1a-4a-7a and 3ab-6ab-9ab are switched, ownership of the units in the shared column passes to a (can be used to trigger SS and to share sp on skills from the new shared units);
- Switch SP (long cool-down): switches sp between a and b, note that this effect only happens after you selected a card to play, so if you have 5 sp, you can not raise lawtia to 5, switch sp and play Bellus in the same turn;
example: both a and b have 3 sp each, a plays Ball of Flame, b plays a lvl 2 and uses switch sp, the gambit is activated before the open skills so both effect of Ball of Flame are activated.
Of course the rules must be properly balanced. (Like finding a solution for Zugateroza in the shared column).