Hello Beak,
You can use the Copy TIN tool with the output extent argument specified to the desired extent, or export the TIN's elements to feature classes, split them using the methodology I posted in this thread, and construct the individual TINs from the resulting features.
Suppose that the Tin data has two peaks and one bottom located between two peaks. After converstion of the Tin data to feature class and classification at specific heignt intervals, I constructed individual Tins from the resulting features.
In this case, it is expressed that portion between two peaks which is actually void is filled.