You can put two 8 ohm speaker in series but best to use the exact same speakers. Make sure you get the polarity correct, left terminal on one to right terminal on other. If you want to double check put a 9v battery across the two driving terminals. With you fingers lightly on the cone they should both push in, or push out in sync.
Positive battery terminal resulting in cone pushing out is referenced at red or positive terminal of speaker. This is a good check if you have have multiple speaker system like in stereo or multi-channel home theater installation. I always do the battery test when using sets of different manf. speakers, just to be sure.
Audio amps are voltage sources so power being V^2/R, given voltage is same, the amp will put out more power if load is heavier with half the speaker impedance. If the amp is not designed for this power and its heat sink on output devices cannot dissipate the extra power it may damage the amp. The amps power supply voltage may sag with higher peak load causing distortion. Also the speaker needs to handle the extra drive power.