WLP002 Attenuation on Imperial Stout

I have an all grain recipe from Austen Homebrew Supply (AHS) for a clone of Stone Imperial Russian Stout. The expected OG is 1.096 and the expected FG is 1.020, and it calls for using White Labs English Ale Yeast (WLP002).

Based on my calculations, I would need an attenuation of around 79% to hit that FG value. White Labs spec for this yeast says that the typical attenuation is 63-70%, so it seems to be a nearly impossible task without doing something else.

I've been reading the "Yeast" book by Chris White, and it suggests that high OG beers (over 1.090) probably need to be aerated with pure oxygen. It also suggests adding a second yeast that is more tolerant of high alcohol and SG levels during the last third of the fermentation.

I don't have the ability to do pure oxygen, but I'm thinking about the option to add a California Ale Yeast (WLP001) after the WLP002 has gotten the SG down to 1.030 or so.

Anyone have any experience and/or thoughts on this?

specs the Labs give aren't really meant to be taken as gospel. wort conditions are more of an indicator of expected attenuation. in fact, i remember someone emailed wyeast about that and they said the given attenuation specs are just guidelines.

i got 70% on a 1.107 imperial stout that I mashed at 158F, using just two packs of us-05 and aerating with a drill whip. if you wanna get down to 1.020 i'd mash around 149

The recipe calls for a mash temperature of 150 F, so that sounds about right.