I wouldn't recommend simultaneous additions, some strains don't get along well with others (read up on competitive factors in yeast). There are two options that I have read about in use:
1) Let the first yeast do it's job until just about complete, then add the second yeast for it's desired effects (usually flavor or higher alcohol tolerance)
2) Ferment the batches separately then blend the finished beer. This can create 3 options; blended beer plus each of the batches separately, or even blended in varying percentages....bench trials are the easiest way to choose your blending percentage.
Just don't add wine yeast to fermenting beer until the beer yeast is COMPLETELY finished. Wine yeast will kill beer yeast and the wine yeast cannot ferment maltose or maltotriose (glucose chains).
Something is always fermenting
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