If my search is well done, there are for now 11 "2-wheeled tractors with trailer" in the IMCDb:
http://www.imcdb.org/vehicle.php?id=82151#VehicleDetails
http://www.imcdb.org/vehicle.php?id=127926#VehicleDetails
http://www.imcdb.org/vehicle.php?id=475838#VehicleDetails
http://www.imcdb.org/vehicle.php?id=491318#VehicleDetails
http://www.imcdb.org/vehicle.php?id=528382#VehicleDetails
http://www.imcdb.org/vehicle.php?id=533378#VehicleDetails
http://www.imcdb.org/vehicle.php?id=622290#VehicleDetails
http://www.imcdb.org/vehicle.php?id=762661#VehicleDetails
http://www.imcdb.org/vehicle.php?id=789311#VehicleDetails
http://www.imcdb.org/vehicle.php?id=929804#VehicleDetails
http://www.imcdb.org/vehicle.php?id=1030282#VehicleDetails
And a possible 12th:
http://www.imcdb.org/vehicle.php?id=430425#VehicleDetails

There's one used by the main characters in a (very) short scene in the series I'm "working" on, so, as these vehicles are apparently used all over the world:
- do we give these vehicles a page, when with a role (other than background vehicle),
- related question: should we then have a specific class for them, as setting a generic "farming vehicles" implies to search them into 172 pages of "normal" tractors (ok, only 2 when one knows that they are identified as "Custom made" in the IMCDb. But who really knows that?),
- or do we just keep them in comment of the movie/series page, as, if they're self-propelled, they usually don't have a seat for a driver when without a trailer?

Not all are listed under the "Custom Made" name, as there were also 2-wheeled factory made models, for example:
http://www.imcdb.org/vehicles_make-Goldoni_model-Special.html
http://www.imcdb.org/vehicles_make-Goldoni_model-Super.html
http://www.imcdb.org/vehicles_make-Ursus_model-C-308.html

I think there was at least one more type, common on the site, but forget of what brand it was and from which country - can't find it now.

All kinds of farming vehicle are lost in the middle of farming vehicles.
If you want to find them more easily you may add something in extra info.
I don't think it is useful to put more detailed categories for vehicles like that, that are more "tolerated" on the site as extra vehicles a little on the edge than being the "core-business" of the site like the normal cars are.

@weasel1984: I never thought we could find the prototype of a 2-wheeled tractor in the IMCDb

Ok. My point was just to have it noted somewhere, so we could refer to it later

I agree on the "tolerated" idea: for me, construction and farming vehicles shouldn't be found in the IMCDb, nor shouldn't most of vehicles from the "Others" category. All the more as they are almost all background vehicles and as they're really far from the "Car" present in the name of the site: one day, this latter will have to be rename into IMMVDb (Internet Movie Motorized Vehicle Database)

I'll certainly set a "2-wheeled tractor with trailer" complement for the one I'll add. I don't know if I'll be courageous enough to add it on the others

for the Custom Made ones, you could even put something in the model field, like we did for Dune Buggy or Sandrail vehicles, that makes them easier to list and find.

You're certainly talking about this one, I think. I "rediscovered" it because "mine" is from the same make