Board Thread:Questions and Answers/@comment-182.53.26.122-20150326131404/@comment-26044227-20150326184636

You might. The problem is that DMM hasn't explicitly said that you could get banned for playing on multiple tabs, but if they think it's ban-worthy, they will ban you for it (the same thing is now happening with people who are refreshing the game too often).

The other danger is if you do PvP and Sorties at the same time. Normally damage sustained during PvP is healed afterwards, but if you are playing both at the same time on different tabs, the damage you take in PvP will not be healed and will show up while you play on the Sortie map. I would strongly recommend not playing on two browser tabs. It's not worth the risk of getting banned.