If you want to share implementation then you need to create a separate **MetacelloPharoCommonPlatform** class/package in a separate package that both **MetacelloPharoPlatform** and **MetacelloPharo20Platform** packages depend upon and that both classes inherit from ... Then methods can be freely added **MetacelloPharoPlatform** without potentially breaking the behavior of **MetacelloPharo20Platform**. There's also to the problem that you can only have one MetacelloPlatform package loaded at any one time so you can't have both classes loaded in the image anyway, without restructuring things ...

Be sure to include a MetacelloPharo20Platform class>>initialize method [that looks like this](https://github.com/dalehenrich/metacello-work/blob/master/repository/Metacello-Platform.pharo.package/MetacelloPharoPlatform.class/class/initialize.st) ... that's the trick that makes the initialize on load work ...