Thanks for your feedback. The current dropbox integration is built in such a way that it will only integrate with a single dropbox account. This was done after consulting many of our SME and accountancy partners and it was found in testing that when integrating one KashFlow account with multiple dropbox accounts led to huge problems for our users, mainly with human error and confusion.
We are not currently looking to change this feature as we find that it works best across our user base. However, if there are a number of votes for the feature we can certainly look at making a change based on the feedback.