There are three options for executing several synchronize tasks. The three toolbar buttons are:

Search for all tasks with files to synchronize and execute.

or by ⌘B shortcut

Do an estimation run and present which tasks with files to be synchronized.

or by ⌘T shortcut

As above, but for the selected tasks.

or by ⌘R shortcut

A double click on a task executes an estimation run and shows the result of the estimation.