I need flexibility due to time as we are not sure what we will be doing in Heidelberg and Amsterdam.
Bit of a chimera, that flexibility. There are 3 direct trains on Sept 30 to Frankfurt airport, 10:40, 12:40, 14:40. The 12:40 can be had now for 160 EUR for 2 1st class tickets, the other two for 200 EUR.
The prices I gave before were for advance bought tickets. If you truly want to maintain your "flexibility", you need Flexpreis tickets and they are quite a bit more, 171 EUR for 2 1st class Amsterdam to Oberhausen.
A 4-day 1st class Twin GRP is 430 EUR. Plus the 2 Flexpreis tickets at 342 EUR, and we are at 772 EUR.
On the other hand, with a little bit of commitment, you can get the Heidelberg to Amsterdam for 240 EUR for 2 1st class, and Amsterdam to Frankfurt airport for 160 EUR.
That leaves those other 2 trips. Frankfurt airport (?) to somewhere, then somewhere to Heidelberg. If it is close by, then 1st class tickets will not be that much. I see that if you can do it for less than 372 EUR, you're ahead of the game.
Edit - If you do as ton suggests, certainly the cheapest way to go, you will have 2 extra train changes, one at Arnhem Centraal, and the other at Duesseldorf. If you want 1st class, it is 39 EUR per person.