With something really big like an RIS you would probably want to pitch two packets of dry yeast anyway in order to get enough cells in there. Using one packet each of two different yeasts vs. two packets of the same yeast probably wouldn't change much in terms of their success at fermenting the high gravity wort though.
As you figured, using two different yeast strains to ferment the same beer should probably give a combined flavour profile somewhere in between the two yeasts used on their own.
58F is really stretching it for most ale yeasts, though with the added heat from the fermentation itself your beer would probably be a few degrees above the ambient temperature. So as long as you kept it somewhere a little warmer while waiting for fermentation to get going you might just squeak by fermenting in the basement.
Having more yeast might start it generating its own heat a little quicker, but I don't think increasing the pitch rate will really do much to enable you to ferment at lower temperatures.
Primary: Beast of Burden hoppy porter/black IPA
Bottle Conditioning: Nothing
Drinking: Buff Face Imperial Stout, Cria (2.X% ABV hop bomb), Llama Doodle NZ hopped IPA