The following article details how to set up custom time graph intervals for PingPlotter Pro, but I've just tested it, and it appears to work in this instance for MultiPing as well. http://www.pingplotter.com/manual/pro/custom_time_graph_intervals.html
In this case, the file you'll want to edit will be MultiPing.INI (which should be located in Program Files > MultiPing). Pull the file open in a text editor, add the time graph intervals as described in the article, and restart MultiPing. After this - if all is successful, you should have a new set of time intervals that include both 7, and 14 day views (or whatever you set your intervals for in this case).
This isn't an officially supported feature of MultiPing - but hopefully the work around should get you where you need to be.
If you have any other questions, or need any more help, please don't hesitate to ask!