You can do writs for multiple cities, but gain faction for only one cities guild. This meaning if I am from Kelethn I can do Qeynos writs and get status, but the guild faction for that city will not go up.
For tradeskilling you can even do TS writs if you are of the opposing faction. All the Kelethin TS writ givers try to get me to craft for them as well when I am from Freeport.
This was just meant to clarify Calthine's answer. ;) I am sorry I could not answer your question though.