I'm not convinced this should be solved like this.
Ivy supports multiple artifacts for a module, maven does not. Perhaps it's better to raise an exception when trying to convert such a module?
Mapping multiple artifacts to multiple maven modules isn't the same!
Using your example: adding a dependency in Ivy on your module is like this:
<dependency org="myorg" name="mymodule" rev="1.0" />
But when the POM is created like this, the dependency-declaration must be rewritten to point to the artifacts directly:
<dependency org="myorg" name="artifact-core" rev="1.0" />
<dependency org="myorg" name="artifact-util" rev="1.0" />
Another approach which maybe could solve some (but not all) issue could be making use of the maven-jar-plugin in combination with the 'classifier' element.
I'm not a maven user, so I don't know if this would help, but maybe it's worth investigating?