You can hook as many subs as you like since the Onkyo only provides the Pre-Out Line Level to the amp(s) that drive it (them). I run two myself.
Bi-wiring is a personal choice and some swear by it while some of us swear at it in a modern home environment. The premise of bi-wiring is that your amps or crossover networks **** to start with so you have to do all this work-around.
My $.02 is you add complexity, waste speaker wire (which can be expensive if you subscribe to the theory that wire can change the sound other than at the cash register, but don't get me started on that), amplifier resources, and MAYBE you'll think you hear an improvement. Perhaps if you lived in an anechoic labratory you might. I think it's a crock.
If you look at the second diagram you'll notice the fundamental electrical circuitry of the speaker wire is essentially unchanged except you now have twice as much speaker wire. The same amplifier output posts still feed the same speaker input posts. How can this be electrically different?
You'll rarely find anyone who can produce a measured audible difference with any of the snake-oil schemes to get into your wallet. It's always flowery prose and faith.