Imperial Stout Fermentation

Hi HBT, this is my first post. I'm a beginner brewer, about to start my fourth batch and would like some input. I am planning a Russian Imperial Stout and am hoping to reach 12-13% ABV. I am considering starting with White Labs San Diego Super Yeast and then adding White Labs Super High Gravity Yeast after several days of fermentation, in order to reach the high ABV. Is this necessary or should I use just one of the yeast strains? Beersmith estimates the OG at 1.098.

This is an experimental batch with the intentions of trying some new aspects of brewing. Any input?

I think using multiple yeast strains can add very appealing complexity to your brews. You even want to consider a third strain that could offer some more esters.
I recently brewed a Russian Imperial and used White Labs London and Burton ale, then 12 hours later added Super High Gravity.
I think your idea of using multiple strains is good though, to answer your questions.
Also, to get the beer up to that alcohol, make sure you keep it on the warm end of things (low to mid 70's) and give a second aeration after about 12 hours.

Thanks for the input. I'll think about adding a third strain, it could be interesting.

I am planning on using a starter for the first yeast but am not sure how to add the Super High Gravity yeast. Would I also do a starter for this? If so, should the starter use DME or would I save some wart for the second starter?

Something else you should consider is that burton yeast can probably get you to 12% (listed up to about 10%) if you make a nice big starter, oxygenate, and ferment at correct temps. You may not need that WLP099, and it may even lead to over-attenuation, which will rob you of the mouthfeel you'll want with this brew.

I would make a starter, and if you were thinking of adding the Super High later on, I would definitely make a starter just to ensure that the yeast is very active. If you have time to plan ahead, make the starters with DME (or wort) and give them enough time to fully ferment and crash out (probably about 3 days) that way you can pour off some of the starter liquid, avoiding adding three full starters to your beer. To get the starters to crash better, give them a day or two to go through a complete reproductive cycle, then stick it in the frig to crash them out faster.

I'm not familiar with the san diego yeast strain, but if its close to chico like its suppsed to be, 12-13% isn't an issue. you're not gunna get to 12-13% if you're only starting at 1.098 tho. I wouldn't bother with WLP099 til you're over 1.120, its not necessary

If you don't have a stirplate, just brew a lower OG ale, rack off the yeast cake as soon as its ready and put your new batch on top