the first one contains all the logic of the user experience, i.e. button, text area and so on, and declare an extension point
<plugin>
<extension-point id="any.id" name="Save" schema="any.id.exsd"/>
...
</plugin>
#################
#### any.id.exsd ####
#################
<element name="client">
<complexType>
<attribute name="class" type="string">
<annotation>
<documentation>

</documentation>
<appinfo>
<meta.attribute kind="java" basedOn=":my.package.ISave"/>
</appinfo>
</annotation>
</attribute>
</complexType>
</element>
the purpose of this extension is to store some data to a datastore.

the second one contains only the logic of implementing the way to persist the data, it implements the any.id plugin
<plugin>
<extension point="any.id">
<client class="another.package.SaveToDatabase">
</client>
</extension>
....
</plugin>
where, obviusly SaveToDatabase class implement the ISave interface.

String saveExtensionPointId = "any.id";

IConfigurationElement[] config = Platform.getExtensionRegistry().getConfigurationElementsFor(saveExtensionPointId);

Arrays.asList(config).forEach(c -> {
	try {
	   ISave s = (ISaveCounter) c.createExecutableExtension("class");
          s.save(...);
	} catch (Exception e) {
	}
});

to deploy both the plugin in the same product, in a monolithic way, and declare the autostart for second plugin, but in this way I did not have the advantages of the repo.

declare some UI object, such as a menu, which do nothing, only to have the Activator started.
Not elegant at all.

If a plug-in contributes an executable extension, another plug-in may run it, causing the plug-in to be automatically loaded. For more details, read the API javadoc for IExecutableExtension in the org.eclipse.core.runtime package.