We've done a lot of the iconic climbs in the French Alps on our tandem (e.g., Alpe d'Huez, Galibier), plus many of the lesser-known ones and many in Switzerland.

The two toughest climbs we've done are the Nufenen Pass in Switzerland, which averages 8.5% for 13 km (8 miles), and tops out at almost 2,500 metres (8,000 ft), see details here. Plus, a tiny road in the Chartreuse mountains north of Grenoble, France, which averaged 10% for 6 km, with a couple of touring panniers (we did have to stop for a rest halfway through, but managed to ride the whole thing). We have done sections of 15-18% for a few hundred metres, and have never had to resort to walking yet.

We can maintain a steady pace of 700 vertical metres per hour for as long as necessary, or we can go full gas at 900 to 1000 metres per hour for about 10 minutes. I use the vertical ascent rate info on our Garmin instead of a power meter or heart rate monitors to know if we're riding a sustainable pace or not. My stoker climbs much faster when the air temperature is below 20 C / 70 F than when it is above, so in the summer we try not to do the big climbs in the middle of the day (when we have a choice).

Our gearing is 53-39-26 with a 12-32 10-speed cassette (700x28 tires); the 26-32 has been just about sufficient for all climbs, and I wouldn't want to go any lower than that because it would mean larger spacing between cogs or chainrings. The bike is not all that light, a Comotion Speedster co-pilot, but we prefer versatility over lightweight.