Hi! I have exactly the same problem... I've been looking for a solution for quite some time :(
I tried implementing "pick random", but it takes only one of the two objects which were supposed to merge together, and destroys and upgrades it. The other one is left as is.
So, in my case, it doesn't solve the issue posted by 
LeKinkss
Do you by any chance have any other ideas on how to work around it?
That'd be great! :)