Hi, a women missed mincha today which would mean to do a second arvit immediately after the first right? But today was rosh chodesh, how does it work with yaale viyavo?

It doesn't help to say a 2nd arvit after you forgot Yaaleh v'yavo in mincha of Rosh Chodesh when it's one day, for you are not going to say Ya'aleh v'yavo in arvit either. In short, you missed it.