How to distribute credit in this situation?
As noted in the comments, it would be best if Alice and Bob didn't try to get either of their names on the object. That smacks of both egotism and insecurity, and is a recipe more designed to create bitterness than scientific progress.

Instead, this type of complexity is exactly what related work sections are for, in which you can say something like:


  Homogeoisopolymorpholisms were first identified in [1,2], then generalized in [3].


and let credit be distributed over time by citation and by ongoing work.  Alice and Bob, as current leaders in this subfield, are in a good position to set a descriptive name, and if they do so and insist that others use it rather than using people's names, then they are likely to have a strong influence on the name that does get adopted.

In contrast, fighting over which person's names are on the object seems likely to damage what appears to be a productive collaboration, and to prevent Alice and Bob from publishing a future set of high impact papers together.