Does biwiring change impedence?

No, he is wrong. Whatever impedance the particular speaker is rated at in non-bi-wired mode, plus cable resistance which should be very small, will be the impedance seen by the amplifier when the connections are bi-wired.

For further background you may find the following link of interest, which I think presents a pretty good explanation of bi-wiring and its potential advantages (although it does not directly address your question):

http://www.brilliancehifi.co.uk/how-to-bi-wire-speakers.htm

Regards,
-- Al

He appears to be thinking of wiring 2 pairs of complete speakers to the same amp. That would halve the impedance. Al has it right about what you are thinking of doing. If I did it I would consider using bi- wire cable with 2 on one end and 4 wires on the other.

Maybe he was talking about strapping two stereo amps to mono and using one amp for each speaker. When you strap a stereo amp to mono the load it sees from the speaker is cut in half.