When you are handed a spoke card, it is your duty to immediately put it on your bike, unless you have radially laced wheels. If that's the case, you can wait until later and use teensy zip ties to attach it to the spokes, at your own risk.
Proper spoke card installation requires a slight spreading of the spokes so that each half of the spoke card is safely nestled between two spokes. Unless you use this method, your spoke card is likely to fly away as you ride away.
I prefer spoke cards to be centered on the non drive side of the rear wheel, across from the valve stem. To each their own, but this is the best method.
If you must double up on spoke cards, place them opposite each other but reading the same direction. And, always, but always, and only, place spoke cards on the bike you rode for the ride the spoke card was designed for.
That concludes today's special pedantic message. See you at Pedalpalooza!