Are you using the right one? There is an 11mm and a 13mm. If you bought the kit the 11mm is the correct one. You really should not need to go that tight. Clean the coupler nut to make sure there is not any grease on the outside surface and give it a snug that’s it.
Just in case. You are not supposed to be closing the gap on the smaller nut, you are only clamping the coupler. The bottom one should has to be loose.