For all others with a similar problem. the function findjobj finds the related java object by its position and size, as described in fileExchange. For that reason the second call of findjobj will return both handles, as they have the same position and the same size. The second call to "set" then overwrites the previous set of Callback with "TAB 1" to the one with "TAB 2".
Giving the object different positions or give findjobj more search parameters solves the problem