Yoby,
You can use two per amp channel... but you don't necessarily "need" them. For Bi-Wire apps you can place them near the output of the amp and connect from the amp to the BB/MC with a short jumper. Then out of the BB/MC you connect both sets of wires as if they were being connected to the output of the amp.
If you did want to use a second set, you could place both pair at the speaker end, one each at the end of the bi-wire pair of speaker cables. Then of course you'd use jumpers to each of the speaker input sections from the BB/MC.
You just have to remember one thing... you only want to use one filter on a given amp output channel at a time. Where would you place it? You would want to experiment for sure, but I'd say to try the filters on the tweeter section first.
Actually, there are a bunch of different ways you can use the Smith Cell section and you can use as many as you wish with no concern for the safety of the amp or the speakers. We have yet to do further experimentation ourselves, but there's a good chance extra benefits can be had by putting a Smith Cell at each end of the speaker wire - amp side and speaker side. Heck... if you had really long speaker wire runs, you could even put a Smith Cell in the middle too.
The reason being that all this might be worth a try is that the effect of the Smith Cell extends both directions down the wires that it is connected to. Theory would suggest that the further the wire gets from the Cell, the less effect the Cell has on reducing phonon modulation of the electron current flow. In other words, as conductor material (speaker wire) becomes further removed from the cell, phonon vibration becomes more random due to thermal energy (heat) and less controlled/reduced by the effects of the Smith Cell.
As I said though, this is just "theory" and it is possible that the additional cells will have minimal to no effect. I guess that would be a good thing though as then we'd all know "one cell does the job" and therefore represents even further value. On the other hand, if additional cells do provide further improvement, then at least we know there is further improvement that can be had if we want to go that far.
If anybody does decide to give it a try, we'd be "all ears" (pun intended :-)and love to hear from you. I wish I had the time to experiment more myself, but I have a lot of other things on my plate right now.
Anyway, I hope this helps. Have a great week all and...
Take care,
-Bob