correct way to bi-wire?

One point to consider is if both the A and the B terminals are wired the same. On my amp the red connector is positive on one set of terminals, and negative on the other. (However, the electrical positive on both sets is marked clearly, if I recall right). This is easy to correct, but only if you know what's what. I don't know what Yamaha does--my arrangement is probably rare--but it's always a good idea to check before assuming anything.

A & B method may be easier since you have two pairs of cables.
In the A & B mode the terminals are just combined inside the amplifier, so I doubt you will notice a difference. They are receiving the same ouput.

Hook up the speakers as you would normally do on one channel ( A ). Play some music as you would normally at a low level. While playing, switch over to the A + B setting with no speakers hooked up to B. If the sound disappears, your speaker jacks are wired in series. If it keeps playing, they are wired in parallel. If the speaker jacks are wired in series, you will have to tie all four wires onto one output ( A ). If the receiver is wired in parallel, you can use A for the woofers and B for the tweeters or vice-versa. While there should be very little difference either way in reality, in theory, the A & B in parralel method would be best. Sean
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Either way is probably fine depending on how the A/B is wired. What the dealer recommends is fine in all cases, so I would go with that to be safe. Probably not the case with your AV amp; but on a small percentage of amps the watts are cut in half per channel if you choose A and B. Just putting them all on A does not increase the load on the amp. It is no different than on a pair of regular non-biwire speakers.

Biwire speaker cables are wired internally, the same way the dealer is telling you to wire the separate pairs.