measure the hole with the old cap , measure with the new........this will tell you how much to mill off the new cap.
If you just take a number to remove from the new piece , you will be disappointed or very very lucky.

I just did the drivers side on mine , but would not be afraid to replace them both.
Having an extra is not a bad idea either.......the typical install is one side and I don't see a ton of benefit to doing both , but it will be stronger.