Hi! So first of all there are actually two types of leveling system for the Ninja Treasured Tools. The first leveling system is the regular one found in the option "Level Up", S tools can get to lvl 75 and this will affect the level up found in "Beset" tab. The "Beset" tab is where the use of Treasured Tool Essences comes into play. Know that the max level of the Beset Level is determined by the tools base level found in "Level Up Tab". I believe if the tools are leveled to level 40, the Beset Level max will be level 9, which will be increased to Beset lvl 10 when base of the Treasure Tools is increased to 50.

It will be based on what you are looking for. If you want to power one specific ninja with the most initiative, then pooling your resources into one is beneficial. However, if you want the most power for the materials you have, then splitting it between ninjas is most useful.

~Jib