I guess we can use Composite Repository to keep all versions available in development update site. It should be declared in compositeContent.xml and contain two links to M1 and M2 release. Below is example from Eclipse documentation
<?xml version='1.0' encoding='UTF-8'?>
<?compositeMetadataRepository version='1.0.0'?>
<repository name='"Eclipse Project Test Site"'
type='org.eclipse.equinox.internal.p2.metadata.repository.CompositeMetadataRepository'
version='1.0.0'>
<properties size='1'>
<property name='p2.timestamp' value='1243822502499'/>
</properties>
<children size='2'>
<child location='http://example.eclipse.org/childOne'/>
<child location='http://example.eclipse.org/childTwo'/>
</children>
</repository>