multiple wp sites, one dropbox account?

Yes, you can. In the settings of WordPress Backup to Dropbox on the left hand side of the your wordpress control console, check the box that says “Store backup in a subfolder of the wpb2d app folder”. Click save changes then a text box should appear to the right of that check box and you can type in the name of the folder you want the site to be saved in. In drop box, you will need to create a folder for each website first. Let me know if that works.