Unfortunately there’s no way to run two copies of the same application on the same Android phone so in order to play with two different accounts you’ll have to manually go to the app settings, clear the settings for the Blood Brothers app and log with your second account everytime you want to play with it and then switch back.
The alternative to this is to run a second copy of the game from your pc with a software called Bluestacks that emulates the Android environment and lets you install and run applications and games quite easily.
The problem with Bluestacks is that it’s automatically set to run in landscape mode while Blood Brothers is a game played in portrait format. This means you’re going to be left with Blood Brothers running rotated on your screen at a 90º angle. Since I’m pretty sure you don’t want to play the game with your head tilted at such an awkward angle the solution is to install an older version of Bluestacks which you can find here. I’ve settled for the last of the 0.7.5 versions which seems to be the most stable one (the earlier ones display some graphic bugs when you run the game). Remember that you have to go to the application settings menu and change the application settings from “Tablet” to “Default” in order for it to display in portrait mode.
Also during the installation of Bluestacks you might be asked to update to the current version of the software and, obviously, you don’t want to do that.
Games run through Bluestacks suffer from fps drop and glitches but since Blood Brothers is not a game where reaction times count you can play it just as well as you would on your phone.