Blau Germany - More than one rate option
 

I purchased two Blau Germany SIMs and activated the 3.90 blau.de-Flatrate option. That way, I have two phones that can call each other unlimited for a month. However, I also want to activate a data option, such as the 9.90 1GB option. I get a message that only one rate option is available during each accounting period. Anyone know a way around this? Would the 2.40/day data option, which is activated by using the tagesflat.eplus.de APN work?

I've just read all the small print on blau's website and also the German prepaid wiki, where I couldn't find any hint on any limitation, that would hinder you from additionally subscribing to a data option.
If you can't subscribe through the voice menu at 1155, call customer care and ask them to activate the data option for you.