If you can make it possible for one to use the toilet while the other is showering, that will eliminate most conflicts.

Consider having a grooming area in their bedrooms, so the bathroom is not used for a 3-hour hair styling session.

I agree that having the toilet separate from the rest of the room is key. I don't think it's as big an issue to have someone showering while the other is brushing teeth (as long as it's not a perfectly clear shower, of course)

I would have a "separate but equal" set up - like many master baths. A stack of drawers for her stuff, a separate one for his. Maybe two shower niches so that she can put her shampoo and body wash in one place and he can put his stuff in another.

I remember many a morning in high school constantly wiping off the mirror while I tried to get ready because I was first shower and my sisters were showering after me and everything was fogged up - so how about a really good exhaust fan or a window that can be opened to let out the steam?

I'm actually working on a new vanity top shared by my three kids and I'm debating having two sinks on 66in vs. one sink. Part of me thinks the counter space is more valuable to three teens, and part of me thinks that the extra sink might be more handy. I've got two boys - and so there may be tooth brushing and shaving going on at the same time. But then again, I've got a girl who will want to spread out I'm sure. So, I can't help with that!

Yeah, I have gone back to the drawing board. I think I've realized that the best thing that could happen is if they both have their own bathrooms. I know they could work it out if they had to share, but I just would rather they both have their own privacy.

But now I'm left with the quandary of what happens if we end up with a third kid. We have never made a final decision about whether that might be in the cards in the future. If I divide the space into two smaller bathrooms, the third kid would make one of them crowded.

Ugh! It is so hard to do all of this remodeling and renovation without a crystal ball!! :-)

Don't discount lazygardens suggestion to have grooming areas in their bedrooms. I'd even suggest that in one or both grooming areas you include a small sink for grooming and brushing teeth. That way at least half of their "bathroom" time will be in the privacy of their own bedrooms, no steamy mirrors while the other sib showers. Sinks in bedrooms aren't uncommon in older homes and in Europe, and they can be very practical. I'm planning on having one in our master bedroom, to compensate for our small single sink master bath. You wouldn't need a large double sink in your kids bath in this scenario, so more room for a separate w/c.

Great suggestions here. My kids are young too, and their bathroom is next on the list of rooms to renovate.