As has been stated 310.4 along with the conductors being smaller than allowed for paralleling, they would also have to meet all the requirements of 310.4
The paralleled conductors in each phase, neutral, or grounded circuit conductor shall
(1) Be the same length
(2) Have the same conductor material
(3) Be the same size in circular mil area
(4) Have the same insulation type
(5) Be terminated in the same manner
Where run in separate raceways or cables, the raceways or cables shall have the same physical characteristics. Conductors of one phase, neutral, or grounded circuit conductor shall not be required to have the same physical characteristics as those of another phase, neutral, or grounded circuit conductor to achieve balance.
Parallel conductors have to have the same characteristics as each other so each gets an equal share of the load.
If you have tied the neutrals from two or more branch circuits together you have the potential for the neutrals to have to carry two or more times the current rating of one branch circuit.
You have tied together the neutrals of two 14/2 NM, 15 amp branch circuits on the same phase together.
One of these cables takes a direct route to the panel, the other cable goes all over the house before getting back to the panel.
Now load both of the hots of these circuits to 15 amps and the two paralleled neutrals have to deal with 30 amps.
Because of the different lengths the current will not split equally and you may have 20 amps on the short 14 AWG and only 10 amps on the longer 14 AWG.
There is also 300.3(B) Conductors of the Same Circuit.