Double Batch One Fermenter

On Day One I brew a 5.5 gal extract batch (1.058 OG), oxygenate the wort and pitch double yeast (2 liquid vials of cal ale no starter) into a 14.5 gallon SS conical. On Day Two (within 24 hours) I brew a 5.5 gallon all grain batch (similar gravity) and add it into the same fermenter to make 11 gallons. The first batch doubles as a large yeast starter for the second. I do not oxygenate the second batch (other than incedental during transfer).

Are there any negative issues with making such a large "starter" (5.5 gals at 1.058 OG) on day one - i.e. growing too many yeast cells too fast, stress, oxidation, etc?

If I were to make an even higher gravity wort (i.e. 1.08 imperial stout) would you recommend making a starter for the "starter" (i.e. day 1 make a 2L 1.04 starter, day two pitch starter into oxygenated extract batch, day three fold in the all grain batch).

Thanks

Ive heard of people making a big batch of beer, canning extra wort in quart jars and adding those every few days for barley wines. Apparently it helps the yeast eat those sugars without dying off. I hear you can get near 25% abv with whitelabs super high gravity ale yeast this way.

http://www.whitelabs.com/beer/strains_wlp099.html

@rimfire: This is exactly what used to be done at the brewery where I work. We have 25hl fermentation tanks and a boil kettle volume of 14hl so each "batch" was split over two days. They've changed a couple of things so that they can do double-brew days to help move things along but there's nothing wrong with what you're doing.

I can't answer to your second question of high gravity brews, the brewery doesn't really do much in that direction and I don't do any personally either. The highest OG brew that they do is a RIS with an abv of about 8.5. No starter, AFAIK.