Mathematically it makes sense and I think almost any chemist would know what you were doing. I don't know that there is a specific rule against it but I strongly suspect that some profs would not allow it. The most important point I can make, however, is that the parentheses are not needed (and that is where the profs that were against it would complain) because they are understood to be there. For example, when we write 2CO it is UNDERSTOOD that C and O are multiplied by 2. 2H2O and it is UNDERSTOOD that both H and O are multiplied by 2 to give 4 H atoms and 2 O atoms. So for you to place parentheses around everything that uses a coefficient is almost like an exercise in futility. You don't need them. They are completely unnecessary. They are just added work and they mean nothing extra that isn't already understood by the rules on equations. Furthermore, I wonder how you would write 2Cu(NO3)2. Something like 2[Cu(NO3)2]. Again, completely unnecessary. The coefficient 2 tells you everything that follows is multiplied by 2, the parentheses tell you that everything WITHIN the parentheses is multiplied by the subscript.
Now for the 2(K + H2O) ==> 2KOH + H2
Frankly, I doubt that any chemistry prof would let you do it, at least publicly, for at least two reasons.
a. It isn't consistent as there are no parentheses around 2(KOH).
b. It becomes more difficult to look at the equation and know that it is or is not balanced. True that it can be done with a little time but it will take extra time and the technique isn't worth the confusion.
You might talk to your prof and see what s/he says. If I were your prof I would do my utmost to deter you from all of the extra work you would be bringing upon yourself. And the 2(K + H2O) thing I would not allow. Too confusing.