I've been thinking about this recently, so I figured I'd ask it here. Is there any reason you couldn't use two strains of yeast in the same batch of beer? For a particularly high gravity amber ale, for example, is there any reason I couldn't pitch 5g worth of American Ale and American Ale II? I've seen it before where people will split a batch, pitch Brett lambicus into one fermenter and Brett bruxellensis into another fermenter and then later blend both batches into one for secondary, but I haven't seen other instances of this with non-Brett yeast. This is more musing than practical, so I anticipate getting some down votes, but I figured I would put it here anyway.
I have done this with good results in the past. There is no reason at all to not try yeast combinations. Yeast is just another ingredient for the most part. Combining two strains to get a little of both character certainly adds complexity to the brew. I have found that when combining something like WLP001 with other strains, you can tone-down the other strains flavor contributions a bit.
Interestingly I combined WLP001 with some Fermentis S-04 to get better flocculation of the WLP001. WLP001 is a notoriously slow flocculator compared to the English yeast S-04. Sure enough the S-04 helped pull down the WLP001 sooner that it would have happened otherwise. I even had two batches side by side with the blend and the solo WLP001. SO I was able to confirm the result.
Definately go for this type of experimentation.
I had some cropped slurry from another batch that was a bit long in the tooth. The Mr Malty calculator said it was only 16% viable, but I had it on hand and it was appropriate to the style, so I made a starter with it and a packet of Nottingham dry. Worked a treat!
Multiple single strain fermentation followed by blending gives much better control over the combined yeast character and is way more reproducible.
It goes like this. Split the batch into multiple fermenters, innoculating each with a different yeast strain. You can split the wort volume equally the first time you brew the recipe. Ferment each part optimizing temperature for each yeast strain (e.g., hotter WLP565, colder for US-05). After fermentation and conditioning, just prior to packaging, you can experiment blending in a small glass with different ratios to reach the desired yeast profile and complexity. When you found the ideal ratio, package according to it and take note. Next time you brew the recipe again, split the wort following this ideal ratio for maximum utilization.