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.

The system was two stereo amps in bridged mono and the owner of the system had them biwired to his 4 ohm Paradigm speakers. I think it would be better to run the amps in stereo and biamp, but that is the way it was. The amps were H/K 16's upgraded and bridged by a craftsman who makes a living repairing and selling audio gear. The seller (the technician) is the one who told both of us that biwiring cuts in half the impedence. He said the amps were looking at 2 ohms since they were bridged and biwired.

Yes, they were looking at 2 ohms, but not because they were biwired. As Racamuti correctly speculated, since the amps were bridged they would see the speaker impedance (in this case 4 ohms) divided by two. They would see the same 2 ohms even if the connections were not biwired.

Regards,
-- Al