While joining two different data sources make sure you should have some identical unique fields in your sources so that you can join the data at granular level. Else you'll get incorrect result while aggregation.
In your case the sample data itself incomplete. Both sources have ID field but with different values so we can not join based on ID field. Data source 1 (DS1) has different timestamp from DS 2. If you see ticket enter date field having date and time while install date from DS 2 having only date values. If you've values at different level then you need to identify the common level from both date fields so that joining can happen.
So, as per sample data you can join both the sources using date fields at Month Year date-part. See the screenshot. Once you established the join condition. Then instead of calc field you can use tableau generated Number of records field to evaluate the result. edit the label in marks card and put ZN(SUM([Number of records])) to handle null as 0.
I don't know whether this solution will work in your original use case or not. If it doesn't work then kindly provide some more data in packaged workbook.