If this is a solid state amp and it is rated for the same power into 4 & 8 ohms then it will not be able to supply the full power that the speakers can take (100 watts). The amount of current the amp can produce will be limited so it won't blow a fuse, go into protection, or blow up.
Higher quality amps will double their power when the impedance is halved. Better to use both channels if possible. If you want the speakers to play the same then just use a Y connector to feed the same input to both channels.