I think you will be fine with a T8 and divider until they get big enough to need their own enclosures. HOWEVER, you will need to quarentine your new snake in another cage before you put your old snake and your new snake in the T8 with the divider.
I woul duse a Heprstat 2 - I have one and LOVE it. If you find you need a RHP for higher ambient heat, then get a Herpstat 4 to control both RHP and both flexwatts
You shouldn't need a fogger. T8s hold humidity quite well