Series vs. Parallel Connection
Unfortunately, getting it right goes beyond the speakers you choose: You also have to wire them correctly. There are two ways you can connect multiple speakers on the same circuit, in parallel or in series.
Connecting them in parallel means running the wire from the positive terminal of one speaker to the positive terminal of the next, and the same with the negative terminals. When you do that each speaker cuts the impedance in half, so two 4-ohm speakers only give 2 ohms impedance.
Connecting them in series means running speaker wire from the positive terminal of one speaker to the negative of the next, and so on. Wiring this way adds the impedances, so two 4-ohm speakers add up to 8 ohms or two 2-ohm speakers add up to 4 ohms.
Ohm's Law and Real-Life Wiring
This is easy enough to follow when you're only connecting a speaker or two. If you have a 2-ohm 6.5-inch mid-bass and a 2-ohm 6x9 woofer, and an amp rated for 4-ohm speakers, you hook them up in series and are done. For more complicated installations, balancing out multiple subwoofers and smaller drivers at different impedance levels, you need to fall back on Ohm's law and do some math.
Ohm's law describes the relationship between voltage, impedance, current and power. It's usually written as V = I x R, meaning voltage equals amperage (current) multiplied by resistance, but you can rearrange it to solve for any of those variables including resistance. You can do the math manually or rely on an online Ohm's law calculator.
Real-world audio systems can include dozens of speakers, so in practice, you might need to use a combination of different impedances and series and parallel wiring to arrive at the resistance you need. If your amp supports 2 to 4 ohms, for example, any end result within that range is acceptable.