Let me quote from the book if it helps any:
"For high-gravity beers, adding a second dose of oxygen between 12 and 18 hours can help fermentation speed and attenuation. The yeast quickly takes up this oxygen uses it for cell membrane maintenance and the production of some needed intermediary compounds. Research also indicates the addition of oxygen at 12 hours increases fermentation speed by 33 percent and decreases flavor compounds such as diacetyl and acetaldehyde. Why wait until 12 hours? You are waiting for the yeast to complete at least one cell division. There is little or no benefit from additional aeration before the yeast has had a chance to divide at least once.
You might also consider adjusting your pitching rate and temperature. For a 1.106 wort, the optimal pitching rate is 35M cells per ml. At 48 hours into fermentation, after yeast growth is complete, raise the temperature to 77 degrees for ale or 68 F for later. The rise in temperature keeps the yeast working at their maximum"
There's a whole section in here on pitching multiple strains of yeast.
I haven't even finished this book yet and have learned a ton. Highly recommend picking this one up.
Yeast-The Practical Guide to Beer Fermentation. By Chris White and Jamil Zainasheff.