been doing some research myself - heading to do west face gully in july (with tmax), I don't think you really need to "acclimatize" per se - I think most do it as either a day hike or camp one night and go. Our plan will be to hike to about 10K camp, summit and head back down to car the next day.
I have not been on shasta yet, but safe to assume you will want to the same you will have on rainier -
this is what I had on rainier last summer - don't take hiking boots, mtnering boots will be drier, - you will be sloshing thru snow most of the time, take a 20 degree rated bag, decent tent to withstand winds, not much shelter up high. take a down jacket - you get cold quick at rest stops,
If dry weather, hike in softshells or other windprooff stuff. - gore-tex is ok, but when the sun comes up, your going to be roasting. even if its like 60 - it will feel like 100!! If it's bad weather, you most likely won't be climbing anyway.
sunscreen everthing - the sun is strong and reflects off the snow - inside your nostrils, ears, etc.. your private parts - yes - this is not a joke, sun reflects up the shorts off the snow!!
not sure your fitness level, but rainier has the potential to take a lot out of you or you will think its a walk in the park -
- so I would recommend doing that after a couple of days rest. Make sure your ready to head up, - its a 9K ele gain in 2 days - at altitude. other than that - I thought rainier was tough - but not as tough as many of the trip reports would lead you to believe. But - I had great weather. Bad wather, I would think it could turn into an unnerving epic - very quick.
one piece of advice would be to start nice and early. 1-2amish on summit days for both mtns. don't head up in bad weather - no matter what you have dealt with here in the northeast -
kevin rooney has done shasta a few times I think - he can probably offer great advice on it.