Merge boundaries

There are some boundaries which should be identical, but got imported and are slightly different in spots. I know I can merge nodes, but there are way too many nodes for that. Is there a josm or other tool that can make this doable?

In particular, I’m looking at these relations, the nat monument is a subset of the BLM
Relation: BLM TresRios (9388961)
Relation: Canyons of the Ancients National Monument (5389886)