I don't think any of the houses are open on Sundays. Visiting 2 houses in that short a period is quite ambitious anyway. Remember - Champagne is to be sipped and enjoyed; not gulped and rushed!

You'll be able to see the Cathedral at Reims and probably take a few pics outside some of the houses if that's your thing. Otherwise I'd recommend just staying in Paris and enjoying that city. As it appears you're into music, I'd suggest going to St. Sulpice for mass. (I'm not religious myself, but mass in a European church is kinda cool.) After the mass there's a free concert (about 30 min) on the massive organ. And after the concert, at the back of the church (on the left if you're facing the organ) an unmarked door will be opened very briefly and you get to walk up to see the organ up close.

When I went the organist was Sophie Veronique Cauchefer-Choplin. She was selling a few of her CDs and was very happy to chat about playing the organ at St Sulpice. (I gather she speaks English quite fluently if you don't speak French.)