Q. Using Blackthorne Pro, can a friend be at his home helping me list items?
A. I doubt that will be a problem as long as you arent uploading your listings at the same time.
Q. Can 2 members be online at the same time?
A. Yes you can be logged into ebay from 2 different locations at the same time.
Q. Can I have 2 stores with 1 username? Im not sure you can. You would have to have 2 accounts.
If you have another credit card you can open a second account using a different username. I do this often to sell specific items on seperate accounts. To give the impressions to buyers that I specialize in only 1 field.