One idea I can think of is just adding several routes and stops pairs to your path, e.g. routes/101,105/stops/4/routes/100/stops/4,9
will give you routes 101 and 105 for stop 4 and route 100 for stops 4 and 9.
Alternatively you could abandon the strict REST concepts and just refactor this into a POST request with JSON something like this: [{routes: [101, 105], stops: [4]}, {routes: [100], stops: [4, 9]}]
. This might also be favourable if you plan to add more query parameters in the future, e.g. getting only accessible routes/stops for certain route/stop combinations.