bad idea for several reasons....

-2 more litres of fuel is going to add a lot of weight to your jet which you have to pack around for the whole flight

-the extra weight will increase the wing loading, and make the plane a real slug to fly until a substantial amount of fuel is burned off.

-the extra wingloading will increase the stall speed, and may well increase the risk of a takeoff accident, or a high speed stall

-without a pump in each wing tip tank, there is no guarantee that they will empty completely, or evenly (even that is no guaranty)

-if one tank emptys more than the other, or you have one that refuses to empty, you are going to have a lot of difficulty keeping the wings in trim, and may run out of aileron control.....

10 minutes is not a bad endurance for a jet..just land, check everything over, refuel and go fly again...............