That's wiring them in parallel, and will, with two 6 Ohm speakers, present a 3 Ohm load to the amp. Some amplifiers, depending on playback levels may not be stable with that low of a resistance, and could go into shut down.
To wire them in series, you run the + from the amp to the + of one speaker, then the - of that speaker to the + of the other, and the - of the second speaker back to the - of the amp. That will make two 6 Ohm speakers appear as a 12 Ohm load. That'll decrease sensitivity (requiring an increased speaker trim setting). Plus the impedance curves of the two speakers will stack, resulting in a less linear response. So this is less ideal than the parallel wiring, but could be required depending on how the amp handles a 3 Ohm load.
The best option, is to run external amps. Split the line-out for the center into two separate amps (or two channels of the same amp), and let each drive an individual speaker at 6 Ohms. There'll be a slight voltage drop in the dividing of the line out, so the channel trim may need to be boosted a little; your SPL meter (or auto setup) will tell you how much.
