Theoretically, you can.

A full time job means 40h/week and two should be 80 h/week. Mentally, it's tough. Well, your friend's company should let you practically work for less than 40 hours.
You need to check with current company's policy if they allow you work for another job fulltime. Some companies don't like this way and you would risk the relationship with them.
Alternatively, since GC can be sponsored for a future position, you don't have to work in your friend's company at this moment. He can apply GC for you for future employment. When you get GC, you should work for sometime in the position you applied in his company. This may be a good strategy to get GC in an easy way.