You will need either an additional formula-field or an in-report Formula to achieve this because you actually want to compare two timestamp fields together based on the date of them being a specific time frame apart.
If you need this formula value in more than 1 report; you should build a formula-checkbox. Otherwise, in the report you have; add a custom-formula and use that in your Filtering section of the report.
If(ToDate([Week Two Follow-up Timestamp]) >= ToDate([Week One Follow-up Timestamp])+Days(7),true,false)
This will evaluate if the Week Two Follow-up Timestamp is equal to or greater than the Week One Follow-up Timestamp date plus 7 days.
