The translation would be: " man yek kaapshan va yek shalvar jin (shalvar lee) ham daram."
"niz" is too formal, you can use "ham" instead of that. And also, you used it in a wrong place in the sentence. "kaapshan" is jacket in Persian. "a pair of" is "yek joft" in Persian and you should know, we do not use it for jeans, trousers, .... but you might see using that for: socks, shoes, slippers, earrings, ... which are found in two pieces.
Good luck with your Persian.