Can I Put Two Activity Symbols (milestone icon, bar) on One Barchart Row?

I want to put two milestone icons or an icon and a bar on one Gantt chart row.  Is this possible.

Example:  We have a important project milestone (or activity) starting in (6) months.  I need to have a planning meeting a minimum of (4) months prior to the milestone/activity.  I hope put the tow items on one row of the Gantt chart to minimize another activity on the schedule as a separate line.  Both activities would be linked logically.  Is this possible?

I have a project that will have 20 such milestones needing 20 planning meetings.  In a perfect world, I could display this data on (20) rows of a barchart instead on (40) rows of a barchart.