Most Hi-Fi amps require a minimum load impedance of 4 ohms or 6 ohms.
Two 6 ohm speakers in parallel gives a total load impedance of 3 ohms. This is below the minimum of 4 or 6 ohms, therefore it is not advisable to connect two 6 ohm speakers in parallel directly to a Hi-Fi amp.
You can use a 2 zone speaker selector switch which uses a series resistor. This will add a series resistor of 3-5 ohms (depending on the manufacturer.) Therefore the 3 ohms (of the two speakers in parallel) and the series resistor (of the selector switch) will bring the total load impedance above 6 ohms, so all should be good for the amp. However the resistor will reduce the power available to each speaker (by up to 60%). See my speaker selector simulator for more details.
You can also simply connect both speakers in series. This will bring the total load impedance to 12 ohms, which will be fine for the amp as it above its minimum load impedance. However, the maximum power of the amplifier will be slightly reduced. For more practical details see my article on How to connect 2 speakers in series.
For more examples of connecting multiple speakers to a HiFi amp see my article on Understanding Speaker Impedance. The video in this article also describes how the different types of speaker selector switches help with impedance.
