here's what i do.. i simply split the wattage of the lamp the are sharing between the diaries so the stats come out proper. even if one isn't quite 25% the overall average will balance out.
it also qualifies you for more contests... so, might as well break them down into individual diaries.
If you put it all in one, you simply make a very large weekly comment.. some headers for each plant would help too. in the end it doesn't matter what you do as long as you get what you want out of it.