I carried one for a while, but then I decided to just use the down tube storage in my Trek Domane. It only fits one tube, but in 1,200 miles of riding, I have yet to have a flat, so it works pretty well. The storage holds one tube, a CO2 canister, a couple of tire levers, a CO2 adapter, and a multi tool.
I realized I don’t like carrying a saddle bag. If I really wanted to carry a second tube, I think I’d put it in my jersey.