Easy question please answer

Ok since i just started rez mod i need help with 1 question. Ok you know how you inject say The UMP. I inject the ump moded file in. But what if i wanted to inject a p90 tre. How can i keep both guns moded but in the same Gun style catagorie? And i need to learn how to do it where as if i downloaded the ump and p90tre seperatly not from one file. Because i know that if you RezExtract a smg file and edit both then inject they would be the same. But what if you downloaded them from mpgh. how do you make them both in one file to inject into combatarms. thanks and hope you pros can answer!

when u inject, after adding the first one, add the second one also. click inject, change .rezNEW ot .rez and profit.