One of the interesting features of tandems of this era is that they are steered by the person in back. Today's tandem bikes are almost always steered from the front. To steer it from the back, the one in your photo has a linkage that lets the rear handlebars move the front wheel. You can see the linkage bar on the side of the frame. That's the steering linkage. The person in front just goes along with the ride as the person in back steers. The people that rode this tandem had to be pretty darn fit because it only has one sprocket on the back and it's pretty small meaning a hard-to-pedal gear, especially if they were to have to ride up any of San Francisco's steep hills.

I'm afraid I can't see anything on the bike that tips me off to the brand or model. I would say that the man is dressed very nicely in cycling knickers, knee socks, leather shoes, gloves and a tidy cap. He appears to have the latest cycling fashions and may have been fairly well to do since the tandem and the cycling gear was expensive even in 1900, and a way to show off your stature in society. I suspect he was riding with another man, though, not a woman, since the front of the bike is a man's frame. If he was with a woman, the front of the bike would have had a sloping frame for the woman to get on and off easily.

Jim Langley | Bicycle Aficionado  www.jimlangley.net
Author of  "Your Home Bicycle Workshop": https://www.roadbikerider.com/