It depends on the speakers and on whatever (receiver/amp/etc.) is driving them. Speakers present a load to the amplifier; that load varies depending on the design of the speakers and the sound frequency. Amplifiers differ on their ability to handle lower loads. Some cheaper amps (receivers) will cut out or shut off when presented with low loads. Others won't mind at all.

So it is kind of specific to what equipment you're going to use. Generally speaking, though, what you want to do will present a load that many amplifiers won't be able to handle.

You might find it will work better to use the existing wires to run a 70v pre-amp signal from your indoor amplifier to a separate outdoor amplifier. Then the outdoor amplifier can control the volume of the outdoor speakers, sepatately from the indoor speakers.