Mutual endorsement, if reputation is conservative, is like giving eachother reputation. If both users reputation is equal, there's no net gain (total reputation is always constant).

If one users reputation is higher, there's redistribution, but algorithms would need to carefully weight reputation linearly in all decisions, making redistribution not advantageous, solving any issues with sybil attacks[1].

[1] https://en.wikipedia.org/wiki/Sybil_attack