In order to ensure the same feature layer is consistently displayed using the same clustering parameters on two different web maps, it would be helpful to have some indication in the user interface as to the value of Cluster Radius.
For example, when setting Transparency for a layer, the percentage value is displayed as you move the slider.
Whether the Cluster radius displays as an actual distance value or as some relative value, the key is having SOME value displayed. Then you can ensure you've dragged the slider to same point in different web maps, so that the same feature layer can be displayed consistently across multiple web maps.
[Workaround: If you're comfortable editing your web maps' JSON data, then you can manually edit the value of "clusterRadius" for the desired layer in each web map, and set it to be the same.]