No, should definately be both TV and rear speakers working together if you have the rears enabled... I think from memory you can also disable the built in TV speakers if you have an external amp, this is the only possible reason I can think of why yours aren't operating with the rears.
Sounds very strange, they're not at all complicated to set up and get working so if you are having trouble I can only suggest that either the rear speakers or the TV (or both) is/are faulty.