Let the twosome play through, of course. Newer players tend to have a rougher time with this.
Generally speaking, I think courtesy in this area is becoming more common. I tend to throw a lot of solo rounds and have run into this a lot - more often than not the group ahead of me will wait and offer to let me play through without even asking.
The one exception - when I'm golfing with my son (12 years, pretty small guy - looks younger than he is) groups almost never want to let us through. There is an assumption that playing behind a kid will really slow them down. They usually change their mind after they see him bomb a couple of shots 300' out there.