Treat each navmesh as a “cell.”
Build a “road network” of waypoints and/or splines between the cells.
When wanting to go from A to B, then figure out:
If A and B are in the same navmesh, then use navmesh only.
If A and B are in different navmeshes, then solve the “road network” using simple search (this can even be pre-calculated.) Then use navmesh A to get to road, use road to get to B, use navmesh B to get to destination.
If A or B are outside a navmesh, you need to find the nearest road or road node, and use that as the destination.
This is a very simple example of “multi-level pathfinding,” which is a rich topic in and of itself 