This speculation has been floating around for years. I remember someone posted a long theory about it (on Burnish or BN, perhaps, I don't remember) about how Chris & Lindsey may have had an affair when he went to Switzerland to play on her solo album.
Personally, I don't think so. However, I loved when they collaborated musically. I loved their singing together - just as much as I do the LB/SN harmonies.