The safe bet is to put them in series, you will do no harm except that the sound quality will suffer from the amp not being able to control the center node between the 2 series speakers. Damping factor seen at each speaker will suffer as well because of the impedance of the other speaker. You may end up with flabby bass, etc.
Speakers are reactive loads, not resistive loads. Each speaker has a motor in it, the voice coil within the magnetic field of the speaker magnet. Velocity of a motor produces back EMF, velocity is directly proportional to the voltage applied. This back EMF is seen in series with the DCR of the speaker to produce the reactance (higher than DCR) seen at the amplifier posts. In a few words, that is why a meter reading of a speaker is not the true impedance listed on a speaker nameplate.
Happy Easter all!