http://www.tp-link.com/en/faq-571.html

TL-ANT2415D is N-type connector. You need a pigtail cable to convert to the Tl-WR841ND

http://www.tp-link.us/products/details/?categoryid=2475&model=TL-ANT24PT3

But the most important point is that what are you trying to do by attaching two TL-ANT2415D to the TL-WR841ND? High again omni-directional antenna like TL-ANT2415D can concentrate the signal at horizontal direction, however the signal will become much weak at vertical direction.