Dates and InfoPath are somewhat tricky. I couldn't figure this out natively within a form. - Various date formulas I've seen on the web are more like essays.
If you're submitting this is a SharePoint library, why not use calculated columns? Then retrieve/change the required date on a subsequent form load?
Columns: ReviewDate (Date initially selected on the form). - Submit form to library.
Then in your list have the following:
Calculated Column - NextWednesday - you could use an excel formula such as - =[ReviewDate]+7-WEEKDAY([ReviewDate],14)
Calculated Column - NextFriday - =[ReviewDate]+7-WEEKDAY([ReviewDate],16)
Calculated Column - NearestWednesdayOrFridayDate - =IF([ReviewDate]>[NextWednesday],[NextFriday],[NextWednesday])
You'll want to play around with the formulas etc. i.e. "with the Nearest Wednesday or Friday Date" to suit your own requirements. If you pick a Friday as you're review date, do you want the nearest date to be that same Friday?
You could also add some text to the form letting users know your change windows days etc.