Can I have backlog items from multiple projects in the same sprint?

No, sorry, a sprint can only contain backlog items from the project that the sprint belongs to.

Note that if you have multiple projects that are closely related, you might consider having them all in the same project in Scrumwise, and then having a separate backlog for each project. This way, you can manage the backlogs individually for each of your projects, but you can work on backlog items from all projects in the same sprints in Scrumwise, and view them together in the same task boards. However, this only works well if your projects are closely related, and it's the same teams that work on all of the projects.