You can keep them separate or combine them.
It doesn't matter in terms of earning and bennies.
If all in one account all the earnings are itemized by card. You won't lose anything. And you retain all the features you mentioned for each card.

Personally however I like mine separate.
When I need more points to redeem then I will just transfer but remember the transfered points expire soon so only transfer when you plan to use them.

Another thing to consider is that once you combine you can't uncombine anymore. Sometimes if you want to cancel a card for example it's just cleaner to have it have its own typ account so you know exactly how much is in there and to clear it out.

So basically I see no benefit to combining in my opinion but others may find it more convenient and easy to track.

But in terms of your question of if they stack....like if you combine, you create some hybrid supercard that earns double and rounds up by 10, the answer is no. Spend with double cash card get double rewards. Spend with R+ and get rounding to 10. No hybrid super card that does both with each unique transaction