Can I mix waterbase Behr wall paint with waterbase deck stain/sealer?
First, are the products chemically compatible?  The mere fact that they're water-based does not make it so.  The best place to ask is the factory.  

However, there's a bigger problem.  Even if they're compatible, a colored paint diluted (what, 2:1?) with a clear coating will have very poor coverage.  If it's going to take 6 coats to get decent coverage on the deck, then why not just paint 4 coats of clear stain, and 2 coats of paint?  

The third problem is that floor paint is a very tough service.  I don't have confidence in any water-based product, but you certainly don't want to use wall paint! For best use of the materials, you might try 1 coat of sealer (to actually seal), 2 coats of wall paint (to color), then more coats of sealer (to provide a durable coating over the color).   That's a stupid amount of work, and I still wouldn't have high hopes. 

If it goes wrong, you may have to remove the failed paint.  That will be far more tedious and expensive than any savings you might have had on paint.

Believe me, I get it.  Nobody wants to waste paint.  I have tens of gallons of paint I have no use for (I am a fastidious color matcher, and I buy a lot of quarts just for color tests).  But my problem is, everything I might paint is too valuable to risk a failed paint job and the huge amount of work to recover from that.  

There's one more problem with newer water-based paints.  The paint industry has done their level best to reduce VOCs, especially toxic VOCs.  The result is nothing poisons the paint from growing microbes or mold.  There have been many cases of people finding paint that's stinkier than normal, painting their walls, and finding the stink won't go away. Covering it with fresh paint doesn't work, their only option is to peel off all the layers of paint down to the failed coat. This is a staggering amount of work that can go into hundreds of hours.  That makes me very reluctant to use old paint at all.  Again, most of my projects are too important for that kind of risk, so I find myself just buying new to avoid any chance of a big problem.