Hi,
Could you please also mention sharepoint version?
For you description, this can be possible in many ways.
1. If you just need to show records from both list: You can use data view webpart to show records from both list without any any code. You will not require to maintain any other master list.
2. If want to maintain master list: Use code to get data from both list and add in master list. Make sure that you have to maintain unique key between both list so you can compare the data. You will need to use SPSiteDataQuery class to get records.
http://msdn.microsoft.com/en-us/library/microsoft.sharepoint.spsitedataquery.aspx
Let us know if you have any doubt
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