Though i have some doubts in your question but i would suggest you to use COMMON CALENDAR so that your data will be able to recognise months as a common field for example on selecting August from common calendar you will be able to filter data from both the tables or you can put months as dimension for your combo chart. I expect month in table 2 is also a date field.
I hope common calendar will help you
You can use the mentioned below link I was following this a time back.
i ll give you a brief also
You need to create LinkTables from you fact table.
i.e for example Table 1 has created on date and Table 2 has a month field. I expect there should also be primary key between these two tables which will help you to connect these table
1. Load Table 1
2. Load Table 2
3. Create a Link table by using Resident (i.e from loaded data) which will have your date field and that primary key. Rename you date field as common date name (any name)
4. Use script mentioned in the tutorial of calling common calendar.
I hope this will help you. Let me know in case of on any doubt.