I wouldn't make it too complicated. Simply say something to your kids that they are growing so big that they aren't going to both fit in the tub soon, so let's do seperate baths! Or if they have different bedtimes, do the younger one first while the 6 yr old is doing "homework" or something and then he gets HIS bath. Maybe you can do a little "storytime" so they still have something to look forward to together.
Good for you with your little one! I'm 38 and have a 1 yr old this month and am thinking about #2, but am a little scared and how close they'll be together and how tired I already am!! But time isn't on my side and my dr said when I had my daughter to have my 2nd in the next 2 yrs...I'm feeling a bit of pressure. It's nice to hear of more "older" moms being able to do it!!