You can be online using more than 1 account at a same time.
I'm talking about 1 account, I think it was obvious no?
Sorry, my bad. I meant more than one device.
Yes, you can login to your account using more than 1 device.
But the answer is Yes, you can use the same account on multiple devices at the same time but if you want to be online you need to keep refreshing the pages for every minute.
Not every minute.
I think the maximum time you should refresh a page or open a new page in to remain online is about 10 minutes.