Is there a reason for storing ground spices separately (unmixed)?
I'm aware of three reasons that you might not want to do so:


You tie up spices that you might want to use in other dishes individually
You don't always want to add the spices at the same time.
You can't always keep spices well-blended.


If you only tend to cook one dish or you leave some of each spice in reserve, the first one isn't really a problem.

The second one is a function of the dish being prepared, which I'll assume isn't the case here.

The last one is less of a problem with ground spices, but can be significant for whole seeds -- as you jostle the container with use, some spices will tend to float to the top, while others sink to the bottom.  The end result is that when you go take your measure of mixed spices, it might not have the same proportion as what you had expected.

You can partially mitigate the issue by taking your container and rolling it around on its side for a bit before righting the container and taking your measurement.  It's not perfect, but it will help in re-distributing the spices.

If there are certain spices that would be a major problem if they weren't help in balance (eg, ground hot peppers), you can blend everything else and leave that one item separate.