I use my bitcointalk account from my device. But my friend can use his bitcointalk account from my device??
You and your "friend" share a computer?
Anyway, this shouldn't be a problem, unless you're going to join a signature campaign / bounty campaign, since the managers might not find your story credible enough.
Bitcointalk.org does not prohibit the use of multiple accounts from one home/device/person, but it is highly frowned upon by some users of the forum, which is understandable ( signature spamming, cluttering up the forum with useless posts, account sellings, scams....)
Which basically means you can't get banned for the solely reason of having multiple accounts, but you could get negatively trusted.