Putting two speakers in series will double the impedance.
It won't cause any problems at all, especially if the amplifier is bridged, because the voltage is doubled and the impedance is doubled, therefore the current stays the same.

It's not common practice, but there's nothing technically wrong with wiring up loudspeakers this way.
It's what the OP asked for, a way to wire two 'speakers to one amp.

Steve.