Currently, when you add any data center as a mudfish option for FFXIV, you cannot then add other data centers. Right now, I’m playing on FFXIV-Dynamis, but I often do a data-center hop with my character to raid on FFXIV-Primal, however I cannot add both items. Switching between the two requires me to delete the item I had and add the other one. It’s a small hassle but one I feel could be improved nevertheless.
Regarding to this issue, please consider to check Create Custom Item link. And FFXIV: A Realm Reborn - Pastebin.com link is an JSON file what Mudfish is currently using. With importing this JSON file, I think you can create multiple FFXIV items on your account.