"I don't have another one just to keep it from you," she advised him, considering he seemed to be implying as much. "It's just because I spend some portion of my time in the Muggle world and it's far easier to have that money there, than going back and forth and exchanging it all the time."
Thinking about it all for a moment, Hermione looked across at Ron. "If I agreed to a joint account here, what would you think of the same thing with my Muggle account?" It would probably take some creativity, considering he didn't really exist in the Muggle world, but she thought she could manage it.