I am no bit expert, so take this for what it's worth.
I think it depends on what kind of mouthpiece the Pelham has. My horse goes in a rubber Pelham and I keep the curb chain really loose. It's a pretty soft option if your horse doesn't need a strong Pelham.
The bottom rein provides leverage, compressing the bridle across the top of the horses head and pinching the curb chain behind the chin. You can control this by how tight you have the curb rein.
With the two rein set up, I can use the snaffle rein as needed and have the curb rein as needed. I will often put a pelham, with the mouthpiece appropriate to the shape of an individual's mouth, on a horse that I get in for rehab until I know how it goes :)