L1 is write through cache, so, whatever changes are made at this level should immediately reflect at L2 level. That is, if a write is made at L1 then immediately a write at L2 should be made. Now, when we calculate the time taken for the write operation, we must consider maximum of the two. Obviously, time to write at L2 level would be greater and would be considered in this case.
Now, this entire process becomes useless. Why did create 2 levels of cache when write operation takes same amount of time in both cases! According to me, this is the reason.
Please let me know if I missed out something!