I just realized that this adapter will not work the way that I wanted to...so now I have a couple of questions.
How can I plug in a J1772 and an extension cord (the one that comes with the motorcycle) at the same time to the motorcycle standard plug in?
You cannot - J1772 provides 220V AC power, and a normal outlet provides 110V AC power (in many countries), and this would cause an electrical fault if you joined them together and probably injury or equipment damage.
I thought I can do it with this adapter... but just realized that I can plug in the J1772 but not the extension cord because both ends, the extension cord and the adapter are the same.
If you're using J1772, the station should have its own cord so that you can bring the plug to the bike, so you will not need an extension cord.
An extension cord is not a good idea for this, anyway, since it could create additional problems delivering current to the bike.
2- Will the motorcycle take two sources of power at the same time ?
You need to buy another charger to use more than one source of power. See:
http://zeromanual.com/index.php/Zero_Aftermarket#ChargersIm just trying to understand the benefit of the 2nd plug (C13) if I can not use an extension cord to plug into and charge the motorcycle at the same time that it is charging from the J1772.
If a cord splits to provide two outputs, the idea is that there is either: (1) another charger that you can use to plug into the Accessory Charging Port above the motor, or (2) a second electric vehicle to charge, or (3) some random electrical load to add like a USB charger for your phone, or some lighting or whatever.