How do I connect two 8 ohm speakers to one channel of an amplifier designed for 8-16 ohms?

It is not possible to make two 8 ohm speakers into a 8 ohm load.

There are only two ways to connect two speakers together, either in series or parallel.

In parallel, two 8 ohms speakers give a total load impedance of 4 ohms – too low for any amp designed for a minimum load of 8 ohms.

In series, two 8 ohm speakers gives a total load impedance of 16 ohms. This will work with amps designed for a minimum load of 8 ohms. However the amp will not operate at full power. In a domestic install, a simple series/parallel speaker selector switch can be used to do this, as well as enable any one pair of speakers to be connected on their own. However, with a 16 ohm load, the amplifier’s output power will be reduced by 3dB compared to using just one 8 ohm speaker.  For more info on this see the article on how Speaker impedance changes amplifier power. 

For the best power output, you would be better off to use just one 8 ohm speaker, provided it will cope with the full amplifier power.

So, it comes down to what you are trying to do. If you want maximum power, then use just one speaker. If you want a wider area covered than what one speaker will cover, then use the two speakers in series.

Also if maximum power is not an issue, then you could also use a speaker selector switch with a series resistor. This will add a 3-5 ohm* resistor in series with the speaker. This will allow both speakers to be wired in parallel (4 ohms total load impedance) in series with the resistor (3-5 ohms) to give a total load impedance close to 8 ohms. However the resistor will take close to half the power of the amplifier.  This is not normally a problem when using multiple speakers in a house, but not so good in a commercial installation requiring maximum power.

*the value of the resistor depends on the manufacturers design.

To see how much power a speaker selector switch resistor takes, see my simulator here.

For a more a detailed understating of speaker impedance, see the article on Understanding speaker impedance.

For more on how to wire speakers in series or parallel, see the article on Connecting 2 speakers to 1 amplifier.