Hi
You can specify which graph an agent should use by passing an additional parameter to the seeker.StartPath method.
Since this is a relatively rare thing to have to do, it is not exposed in the Unity inspector.
// This is the index of the graph that you want to search on
// The first graph in the graphs list will be at index 0, the second one at index 1 and so forth.
int graphNumber = 2;
int graphMask = 1 << graphNumber;
seeker.StartPath(startPoint, endPoint, null, graphMask);
See also AI of different size in Recast Graph
See also https://arongranberg.com/astar/docs/class_seeker.php#ab50a876ab529ceb3eb9b97deb5a48d1e