It is very difficult if not impossible to control how the system processes logged caches. Logging them in a certain order does not ensure they will receive a date / time stamp conducive to order one before the other. What is more likely to happen is the server will process based on meta data beyond your control.
The only hope is if a control is programmed that actually allows you to override the system and manually choose a cache over another as your milestone just like geocaching.com provides. Until PGC implements such a control, don't expect to be able to control milestones.