By the way, what amp are you planning on using the drive the speakers hooked up to the mixer? Unless it's a mixer with a power amp (pa) it won't drive them. Also don't try hooking them up to the amp unless you check impedance. While many solid state amps have a range of tolerance, speaker impedance mismatch can burn it up depending on its design. Keep in mind that when hooking up two speakers in parallel the impedance is cut in half, whereas hooking them up in series will double it.
So if the amp is 8 ohms and the speakers are 4 you can hook them up in series and the amp will see an 8ohm speaker. If they are 16ohms you could run them in parallel and it would see an 8 ohm speaker. But if you tried to hook up two 8 ohm speakers it could only be 16 or 4 ohms. Again some 8 ohm amps can tolerate a 4ohm speaker (and even gain some power due to less resistance) but it's important to check the specs of the amp. If the ohms of the speakers aren't listed on the speakers you can check it with an multimeter (ohms setting). Many solid state amps are rated at 8ohms but can also run 4 ohms. 8ohms is fairly common for auxiliary speakers (but not always the case).
Another option is a small powered mixer.
As far as the cabling you'd need to hook them to the amp: It depends on how the amp's current speaker is connected. Many plug in with a 1/4" jack (same as a instrument cable jack) but don't use an instrument cable to connect speakers. For low volumes this could be fine but if turned loud enough it can fry the cable (mostly an issue with tube amps as they can burn out when they aren't seeing a speaker load). If there is just a wire running from amp to speaker you'd have to connect them to that. One more note: If you keep the speaker of the amp connected you have to include it's impedance in the equation. So if it's 8 ohms and you have two 4 ohm speakers you could hook the extension speakers in series and hook that pair in parallel with the amp speaker and have a total of 4 ohms (probably within the safe range for your amp.
If all of that is confusing, tell me the amp type, speaker model number and I'll look up their specs and figure out the best option for wiring them.
One Ring to rule them all, One Ring to find them, One Ring to bring them all, and in the darkness bind them.