I'm not, but I've read that doing a 2nd activity is a very good fitness goal as it drives the metabolism up, burns calories. etc.... I think it has to do that a morning activity will be driving up the metabolism for a few hours after you stop, doing a second later in the day keeps the metabolism high and that's beneficial. It was good for me when I was bike commuting, the twice a day ride had me losing weight. I also will sometimes swim in the afternoon if the morning ride wasn't too hard, as my legs get very tired and tend to cramp in the pool if the bike workout was hard. It's also harder in some regions to cycle in the afternoon if it's hot, and I rarely get an opportunity to do a dusk ride, which is a good time to ride.