We did Les Chefs (12 noon) and Le Cellier (7:10) on the same day last year, except our ressies were the opposite of yours--lunch for Les Chefs and dinner at Le Cellier. While we enjoyed both A LOT, we were kind of, no not kind of--absolutely, miserable after Le Cellier. All of us said we wished we had eaten lighter/less/not gotten so full at Les Chefs (we wished that the three of us had shared two of the three course fixed prix lunches--so I agree with PP that's a great idea) because we were still not truly hungry when we got to Le cellier and we did not feel we had enjoyed it as much as we might have otherwise since we really weren't hungry yet. We did not even want dessert at Le Cellier. But like you, I couldn't bear to give up either one, so hopefully this helps you have a better game plan than I had.
But your ressies have Le Cellier first, so I think you will be fine, especially if you share at Les Chefs. There are a lot of fun things to try at Les Chefs, so sharing opens up more opportunities! (My little 8 year old "foodie" said "Oh Mommy tell them to try the Onion Soup and the alsace flatbread!!! And the macaroni and cheese. And the cheese plate. And..." LOL).
I say go for both!!!!