Raging Rivers

Here's a Raging River:

The Oracle text for the card is the following:

Whenever one or more creatures you control attack, each defending player divides all creatures without flying he or she controls into a "left" pile and a "right" pile. Then, for each attacking creature you control, choose "left" or "right." That creature can't be blocked this combat except by creatures with flying and creatures in a pile with the chosen label.

This makes gameplay interesting. Basically the battlefield is divided into two, and ground creatures (without flying) can only attack and defend on one half of it.

Question: What happens if there are two Raging Rivers on the battlefield at the same time?

This situation is a bit complicated. While it's not explicitly stated in the game's rules how exactly the situation should be handled, it can nevertheless be deduced unambiguously from how effects work.

To analyze this situation, let's assume that there are six Grizzly Bears attacking and six Civic Wayfinders defending. (It's not all that important what the creatures are for the sake of this example.)

When the bears attack, both Rivers will trigger, but they will resolve one at a time. When the first River effect resolves, the attacking and defending creatures may have been divided into "left" and "right" groups for example like this (you can click on the image to get a larger version):

I have color-coded the effect of the first River with yellow markers for clarity. What the effect in question is doing is creating a continuous effect (during this combat phase) that imposes a restriction on which creature can block which other creature: Those creatures marked with a yellow L can only block those attacking creatures with a yellow L, and likewise for the yellow R markers.

Now the effect of the second River resolves. All creatures are now independently divided into "left" and "right" groups due to this effect. I have color-coded this second effect with purple markers. They could be divided, for example, like this:

The second effect likewise now states that only creatures with a purple L can block attacking creatures with a purple L, and likewise for the purple R.

The division into two groups for the second (purple) effect is completely independent of the first (yellow) effect. However, both effects are still imposing a restriction on which creature can block what. Both effects have to be taken into account when declaring blockers. For example, even though the second effect would allow a creature with a purple L to block an attacking creature with a purple L, the first effect will be restricting that possibility if they have a different yellow marker.

Therefore the only possible way to declare blockers is like this:

So, quite flavorfulwise, two Raging Rivers will divide the battlefield into four parts, and all non-flying attackers and blockers will be restricted to one of those parts. It's like a river that splits into two, and there's a mountain range between them.