im of course new to the brewing business but loving this new hobby! i have probably been too enthused about it. I have a question about mixing different strains of yeast. i have some collected leftovers in an Erlenmeyer of west yorkshire and london wyeast from my bitter and pale ale batch and i was pondering pitching them in the same new batch. ive looked a bit(maybe im blind) but cant find too many suggestions on it. what usually occurs with mixing?
Mixing the yeast will not hurt anything.
You just won’t know which yeast will dominate and what characteristics will come through.
It will just spice things up a bit, blending characteristics of each. If I had to guess you’d get some more of the West Yorkshire nutty/fruity flavor characteristics with the higher attenuation of the London which would reduce the chewy malt flavor but it’s kind of a crap shoot at this point without really know how much of each is going in there. here’s what the website says about each.
London:
A rich mineral profile that is bold and crisp with some fruitiness. Often used for higher gravity ales and when a high level of attenuation is desired.
Yorkshire:
This strain produces ales with a full chewy malt flavor and character, but finishes dry, producing famously balanced beers. Expect moderate nutty and stone-fruit esters. Best used for the production of cask-conditioned bitters, ESB and mild ales. Reliably flocculent, producing bright beer without filtration.
Do it, and don’t sweat it! 
It’s true that one strain will dominate and be something of a wild card, but if you do everything else right, whichever strain dominates the fermentation won’t matter …and you’ll wind up with good beer.
I’ve done this very thing number of times and was never, ever disappointed with the end result.
Just do it.
Experimentation like this is the best part of the hobby
There are several yeast blends sold by WLP and Wyeast, if they didn’t sell they would be pulled off the market.
