My daughter will be 20 months old when her brother will be born this coming May. She has been EC'ed since 4.5 months old, but is still nowhere near stage 1 (dry with my help).
What my plan is now, is to keep her in gerber training underwear (cotton waffle) and do as much EC as I possibly can during now and then, in hopes that she'll be able to more or less keep herself dry in 4 months from now. She's definitely building awareness, but regardless of where she's at come May, I am expecting some regression when the baby is born.
I plan to do 100% diaper free with the baby (except during times when he'd need protection - like in the car), which likely means I will be wearing him a lot. I plan to never use disposables, and am already stocked up on prefold diapers, which I will use without a cover (except in the car).
Kids around that age love to mimic and help, so having your first child help with pottying your second, might even speed up her own toileting abilities.
I'm a little nervous about taking on too much, but the more I think about it, the less I stress. I think it will work just fine.