Each amplifier lists what impedance the rater power is for. So, in order to connect two speakers to single-channel amplifier, you have two options (let's assume your speakers are 8Ohm):
- connect them in parallel, thus reducing their impedance twice (so your 8Ohm speakers will become one 4Ohm speaker)
- connect them in series, thus doubling their impedance (your 8Ohm speakers will become one 16Ohm speaker).
So, check with your amplifier what it can support.