Double lesson cannot be on Monday (specific days/periods/part of the day)

For this constraint use the menu "Main - Relations - Advanced".
With relation type "Max periods per day" you can simply say, that there can be 0 periods (3.) on day selected in time-off map. There select Monday only (6.) (or other required possitions, where you do not want your lessons to be) .

Important for this specific case is to use the "Options" button (7.), where you can select, that this rule shall only apply to the lessons of length of 2 periods