The entire gouge can be found here:https://www.geocaching.com/about/guidelines.aspx#event
Here's the pertinent part:
CITO Event Caches
Cache In Trash Out events are gatherings of geocachers to improve parks and other cache-friendly places. Examples of CITO-appropriate activities include tree-planting, trail-building, removing invasive species and removing trash from a designated location. Other organizations sponsor similar activities. These external events could be adapted or developed to meet our cache submission guidelines. To be published on Geocaching.com external events will need to designate a portion or section of the larger event to be by geocachers for geocachers.
CITO Event listings must include a start and end time and last at least 1 hour.
I'd avoid trying to coordinate between two organizations, and just post a CITO event and run it in parallel. I'd do only one day (probably the first day they have scheduled), and either morning or afternoon. 0800-1100. Choose day one, as there will be more trash, and it will be more rewarding. Hope this helps.