Bolt shear is a limit state for the bolt, whereas bearing stress is more a limit state for the connected plates. Therefore, it makes sense to evaluate bearing stress on a plate-by-plate basis. When two or more plates are sharing the load, you can consider the total thickness in each loaded direction versus the total applied load.

I suppose that in principle, when two or more plates are sharing the load you could split the load between the plates based on relative bearing area and evaluate adequacy on a plate-by-plate basis. However, since the loaded 'width' is the same for all plates (i.e. the bolt diameter) this approach would give the same result.


A little qualitative thought experiment:

[![Bolt Bearing][1]][1]

 - There are two shear planes and three bearing 'areas'
 - The stress on the two shear planes is equal
 - The $Plate_1$ load + $Plate_2$ load = $Plate_3$ load
 - Load in $Plate_1$ > load in $Plate_2$ (because $t_1$ > $t_2$)
 - Bearing stress at $Plate_1$ = bearing stress at $Plate_2$ = $\frac{P}{bolt diameter(t1 + t2)}$

 - If ($t_1$ + $t_2$) = $t_3$ then the bearing stress is uniform along the bolt length, otherwise it will vary


  [1]: https://i.sstatic.net/e0wxn.png