There are few trunk holders for NIU N-series that I know. We at shop have 2 original versions, first one is stylish aluminium and enchanted version which is much more heavy duty. Also we have aftermarket SHAD holder for NIU which looks similar to stylish aluminum original version.
Now if you want to fit 2 helmets in trunk I would recommend you to get enchanted version original holder and aftermarket top case (trunk). Shad 39 would be smallest top case that we could offer to our clients for 2 helmets (one full face and second jet). Also I would like to point out that NIU would look very silly with that big top case and you would for sure loose some range. But if it is necessary that is one solution.