I'm afraid you will have to wait for famillies for this one.
Having the drops belonging to two famillies (like A and B) and checking for collision between A and B.
That's the "easy way".
After, concerning the "trickier way" it will depends on your game and how you implement it.
If the drops are falling (vertical moving down) drops that are spawned "in line" (not randomly, but one after the other on a progressive X) you might then consider that when two drops collides, they are "neighbours".
Considering a "for each objects" loop runs through the instances in incremential order, you could set a system in which when a
drop is colliding with an other
=> Have a temp variable incremented (for exemple nbDrops)
=> Store the UID of the drop in an array or some global variable (DropOne for example)
=> Store the values that you want (height, width, ...) the same way (an array is advised to store all in one line, you will need two X and as many Y as you have values to store)
Test the value of nbDrops (subevent)
.. System nbDrops = 2
..=> Set nbDrops to 0
..=> Create the new object Drop and set its size by adding the size values contained in your array.
Subevents pick Drop by uid
.... Pick the first UID of the array
....=> Destroy drop
.... Pick the second UID of the array
....=> Destroy drop
.... blank subevent
.... => Clean array
This works on the condition that drops are nearby and that two colliding drops will have close/following UIDs.
If you spawn drops in random positions you might end up with full mayehm of distant drops disappearing and bigger drops appearing widely/randomly on screen.
If someone knows a good way to implement "recognition/separation" in the collision of two instances I'd be glad to hear it (@r0j0hound, Ashley, ideas ?)