http://revach.net/article.php?id=1878

Section: Halacha   Category: Tshuvos
Pri Megadim/Minchas Yitzchok - To Party or Not on Shushan Purim Katan?
In a year where there are two months of Adar, the Rema says (OC 697:1) that some say that you should have "Mishte ViSimcha"; make a party and be happy even on the 14th day of Adar Rishon which is Purim Katan.  The Mishna Brura says from the Pri Megadim that even those who hold of this minhag on the 14th agree that on the 15th, Shushan Purim Katan, we do not party, even those who live in cities that celebrate Purim on the 15th.

The Minchas Yitzchok says that others hold that the Rema was talking about people who normally have Purim on the 14th.  They should not celebrate Shushan Purim Katan.  However he was not referring to people who live in cities that have Shushan Purim.  They may still have a mitzva to have Mishte ViSimcha on the 15th day of Adar Rishon.  

Disclaimer: We try to convey the Tshuva to the best of our ability. We admit that our understanding may not be accurate. Please also understand that this Tshuva may not be the final word on this topic. One should consult a Rav before drawing any conclusions.