Yes, you can stack heightfields on top of each other.
One way would be to merge the green Heightfield Load nodes together via the Heightfield Merge node. You would still be able to apply Heightfield operators, like Erode v3, to each individual heightfields.
You could also chain together the red Heightfield shader nodes (Output to Main Input), or use a Merge shader to connect them.
If you're only seeing the effect of one Heightfield, it may be due to the Flatten Surface First checkbox being enabled under the Displacement tab of the Heightfield shader node.