According to Eshragh for 278 and 279 H coins with both names are also known from Basra, Kufa and Surra man Ra'a (B1353, B1354 and B1356). The obverses have "al-Mu'tadid billah" and the reverses "al-Mu'tamid alallah". Interestingly, Eshragh doesn't list a coin of this type from Samarkand, so there may be other mints that used both names as well, but seemingly just in 278-9.
Alan