Of course they can, not only are different studios working on them, but they are different gaming markets as well. Some people prefer SP games and some prefer MMO games, the best thing would be to try to make products for them both.
The market is the same I assure you , maybe except for a very small part which will not suffice to make a succes from a SP game. An MMO is supposed to be played for many years in order to make as much profit as possible, thats why I dont expect another TES game for a long time if ever again.