I think this could be a good idea that also includes the swarm concept noted. I perceive this more as a creature trait, separate from conjurations. A modular conjuration effect could be an interesting adaptation as well, but covered by it's own traits.
For creatures, a trait that conveys the sense of the mercurial - the ability to blend and separate at will. Two creatures of the same type that share this trait could stack together, one adding a specific bonus to the other as noted by the OP. Attack, armor, life, status effect, etc. Not it's whole block of stats, just something specifically noted in the creature text box. The creatures would be of the same type, but could be different in name. For example, the Straywood Leafcutter and the Wytchwood Bullet Ant are both of the Ant type, and share this "melding" trait, so could conceivably stack with one being the primary creature and the other being the secondary that adds a bonus. Separately these creatures might be fairly weak, but stacked might be quite powerful, and certainly flexible enough to respond to board conditions.
This could cover creatures like oozes, insects, undead, wisps, elementals, soldier units, etc.
Joining would be a quick action performed by either creature. If it is the secondary creature, then the primary still has an action free to use with it's new upgraded status. If the primary has already acted, then the secondary can join it then wait until next round. Separating would also be a quick action per creature, but they would have to wait until the next round to re-join or join another collective. If the primary creature is killed, all elements stacked with it also die.
Of course one drawback to all of this is the absolute mess it could potentially make of your board, but realistically should be no worse than a creature with a cartload of enchantments.
For conjurations, I see two compatible modules with one being cast onto another increasing the overall effect of the module - say a spawn point to which you can install a source that increases grants or increases channeling, or a life-giving tree with an added essence which increases the regen for friendly creatures.
[/spitball]