The yeast I am currently using is 4th generation harvested yeast, and the original was a mix of HotHead and BE-256. The beer was a bigger beer than what the single pack of HotHead was supposed to be good for, and it was a little old, so I also pitched a packet of dry BE-256 sort of just in case. Very strong ferment, rapid start, quick attenuation and no blowoff though it would have been close with a 6 gallon batch. For the next brew, I poured the next batch of wort right over the old trub, From there I just innocculated each successive batch with saved yeast, and this current batch I made a one quart starter the day before brew day. I think that the BE-256 has become the dominant strain in my yeast and that is perfectly okay. I am getting very rapid starts and about 3" of krausen but it never gets out of control, and the results are great. My beers run at least 1.070 OG and usually 5 to 20 points higher than that.
I have started using a starter so I know positively that the culture is still viable. I think my starter base was a half pound of dark DME in a quart plus a few ounces of water, boiled for 10 minutes and cooled to 80F before adding the yeast straight from the fridge. It was a pretty big pitch of yeast so I wasn't too worried about survival rate. It took off in a couple of hours and I knew I had a good starter for the next day's brew.
The original batch is on tap now, and it's a good one, so mixing those two yeasts worked out just fine. It's not very dry, but that isn't usually a goal with me. I like a very chewy and filling brew. This one has no funny smells or tastes, just a solid liquid bread experience, plenty of ABV, nice persistent and sticky head.
I doubt if you will very often see two yeasts combine to make a bad beer if both yeasts were originally good strains with no contamination. You might often see that the combination is superior to the sum of the parts. When you think you got to pitch two yeasts, then do it, as long as you can maintain temp within the range of both. However, when in doubt, you can always make a starter and if there is no action in the starter overnight, then do a double pitch. I didn't do that though, and it still came out great.
