Yes, Of course you can use 2 A/c in one time on same IP but you can use to browser. Link Mozilla for first account and Internet explorer for second account.
As long as they are under different names you will be fine using them under the same IP. However if they ever deem one of the accounts as suspicious and limit it... they may limit all the accounts going to that IP. Especially if they are selling something in the same market such as all of the accounts selling shoes on ebay, that would look pretty suspicious to them and they might hit you all with limits. If they are just personal accounts, and you don't really sell much, but us it to buy... have no fear your safe haha. Also if your use good business practices while you sell you should be fine also.. I lived with my brother and we both sold xenon lights on ebay. My account ended up getting suspended over lamborgini door kits, his ebay and paypal were fine though. He had also been limited by paypal before though. Worst case senario (as long as your not doing shady business) you may have to send them some info like photo id, and they may hold your funds until you do so.
Yes, definitely problem. Never use same ip when using multiple Paypal accounts. Always switch ip. If any reason, one of your Paypal account limited, the other account used the same ip also get in trouble, no doubt.