Firstly you need to create a similar field using a calculated field that tableau can use - I'm not good with dates but some googled instructions that look good are here Date Formatting - YYYYWW
Once you have created the calculated field, blend your data using the date field from the 1st data source, and the calculated field from the 2nd data source. You should then be able to use this in an axis.
This does not work because once i add both the dates to my graph..it changes from a line graph to a dotted line graph
Can you share either your packaged workbook or your data and a mock up of what you need so that we can respond accordingly?