I honestly don't know the legality of this set up, but it does not increase the capacity, same if one were to tape 2 or 3 mags together for quicker reloading, however it's the wording in the statute below that is not specific.
According to our criminal code, specifically section N.J.S.A. 2C:39-1, a large capacity magazine is defined as a box, drum, tube or other container which is capable of holding more than 10 rounds of ammunition to be fed continuously and directly into a semi-automatic firearm. Thus, is it unlawful for any individual to possess a magazine that holds more than 10 bullets. Any person who knowingly has in his or her possession a large capacity magazine is guilty of a crime of the fourth degree. N.J.S.A. 2C:39-3j.
There is nothing in there that states one cannot fix two mags together unless it's somewhere else in the laws, but the part in red begs the question from us, do two magazines fixed together count as one ?
There are so many gray areas in NJ laws, and most are not specific, perhaps someone with more knowledge of NJ firearms laws can chime in.
I can change mags quicker than the 2 mags held together by the base plates, I suppose it's the muscle memory for me.