A few weeks ago we got a potty chair for the babies. We decided that at 18 months we would ease into the whole tee-tee in the toilet phase. We aren't going to push them, but rather just let them explore sitting of the toilet and see how things go.
Well, yesterday and today Mazie has tee-teed in the toilet. This afternoon, when we arrived home from school, I took her diaper off and told her to go tee-tee in the toilet. She went in the bathroom and I started undressing Lucia in the family room. I heard Mazie say "See, see." Then she came out into the hall and said "See, see." She had indeed tee-teed in the toilet.
We will keep exploring with the pottying and see how it goes. We know they each will get ready in their own time.