That's right.
I think the sequence of 1st / 2nd clicked item should not effect the final unit.
which put player focus on the sequence is not that needed and is not shown anywhere.
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And, If both items have addon.
I think you can keep the one of the target, which is current logic.
and you can give a hint on the (target) to show that addon is to be kept. (like an green outline of the addon or something) , also a small drop out anim on the dragging unit's addon may be good