Samenreiskorting is not for one day, but for one ride. You need to activate it for every journey between check in and check out again.

It remains valid until 35 minutes after a check out, and it is possible to check in again with reduction in that situation. But this does not apply to you.

The fare is correctly calculated.

Advice: You can activate ‘Samenreiskorting 2x’ when you do use Samenreiskorting again to travel back and forth, as you did. 

It's a bit confusing but @Thom is right, see https://www.ns.nl/en/travel-information/traveling-with-the-ov-chipkaart/combined-travel-discount.html

The discount is valid all day until 04:00 the next morning or the moment you check out.

That should read ‘is ready for use (check in outside peak hours) until 04.00 the next morning, and expires 35 minutes after check out’.

Thank you Thom and Robert. 
 

You’re welcome!