Say I wanted to make an ale which has some of the banana characteristics of a Belgian yeast, but also wanted some of the citrusy flavors of an American ale yeast. Could I achieve this by making a starter which is half Belgian yeast and half American yeast (or maybe some other ratio)? What results should I expect from introducing two (or more!) strains of yeast in the same batch?
I do not know of any American Ale yeasts that provide citrusy flavors. Its all hop derived. American Ale yeast is very neutral in flavor. So yes you can mix them, but the only fermentation character you will really detect will be from the belgian strain.
I have done some interesting blending of English Ale yeast and American Ale yeast in the past. The flavor was all English in character, just mildly subdued. I used two yeast strains I was very very familiar with so I can say subdued with confidence.
But the reason why I blended them wasn't for flavor it was for flocculation. I was wondering if the super flocculant nature of the the English yeast would help to pull down the American strain (which normally takes much longer and cooler temps to flocc well).
And yes indeed all the yeast flocculated out quite nicely. It wasn't 100% like the English would be alone, but man it was really improved over American alone.
The strains I used were WLP001 from WhiteLabs and S-04 from Fermentis.
Good luck with the experiment.
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.