We propose in this paper a new online Multi-User Multi-Touch handwritten diagram DataBase (MUMTDB) for evaluating recognition systems under the multi-user situation. The data is collected according to two predefined mind map scenarios which contains 9 classes of graphical symbols. Each scenario is completed by involving two users at the same time. Since the users are given freedom to draw the symbols as they want, the dataset contains a diversity of multi-stroke and even multi-touch symbols. It allows addressing new challenging problems regarding the recognition of simultaneous composition of structured documents. The dataset is freely available on-line.