Yes you can be logged into as many devices as you want and be simultaneously running transactions.  All together we run on 3 to 10 devices at the same time.

Edit: Didn't see this was answered already.  anyway, it does work.

Yes it looks like any devices that have the same login will automatically use the same register layout and bank account linked.  

Another question to the square community.  Say I want to run two separate business.  Is there an option to switch between different layouts and bank deposits?