Typically you can have only one "set" of speakers connected to a system at a time. Most mobo's and sound card's only have one input section, it may have multi-ports but it's still just the one input section. There may be an adapter to connect to split the input panel into 2 but I've not seen one.
Your only option is to get a 5.1 surround speaker setup if you want use more than two speakers.
As a work around and I don't suggest doing it, but you could splice the 2nd set of speakers to the the first set by cutting the wires directly and attaching the 2nd set that way. I did this back in the late 70's and early 80's before there was surround sound to get 4 speakers working.