Weapon merging as you define is simply infusing a weapon with the power level of the other. What you're describing seems to me like the feature itself, but in reverse. If you want to keep a weapon that has good perks, lock it and infuse it later down the line.
I don't think there'd be a point to have a system like this, because it kind of already exists.
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For example: I have two Night Watch I want merge on same weapon power level and perks At not same weapon merge only power
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Yes, system already exists, but i say about evolution of system, or next level. Because current level is not good.