Your buffalo is 40mW out. Your splitter probably throws in a 1-2dB loss so about 25-30mW then split, with 3dB down on each output, then output in to a 24dB gain antenna giving you an e.i.r.p somewhere near 3300mW before cable loss.

Take some cable loss in to account, you're probably around 3000mW (1000mW below the legal limit).

If you include the 6dB antenna on the same splitter, however, it makes no real difference since it'll only be about 40-50mW output.

The grand total e.i.r.p will still be shy of 4000mW. But without a watt meter to confirm what each antenna is getting, you won't know. No computer model can give you the perfect answer but it does sound like you under not over.

Gavin.