Translation:Are you wearing a hat and a t-shirt, Seth?
Well, as this is not really an Arabic word I think both versions are acceptable. However English compound words are usually split in Arabic (e.g. Kickboxing = كيك بوكسينغ), especially when the first part of the English word only consists of one letter (إكس بوكس = Xbox) or when we are talking about an abbreviation (واي فاي = WiFi). As for T-Shirt it is written تي شيرت in my three dictionaries and also in the Arabic version of the Wikipedia entry about T-Shirts. So I guess it's OK.