Every tailor I have known runs two lines of stitching on the seat. One on top of the other. There is a special nylon thread that has a natural elasticity to it that is designed to use only on the inseam and waist/ seat seam. We use this on certain cloths.
What you are describing doesn't sound like a comfortably cut trouser
Depending on the tensile strength of the cloth, I would be concerned about the cloth tearing rather than the seam splitting.
Thanks; it's Dugdale's Cape Breeze mohair blend. My tailor just told me to be careful. I do request a slim fitting trouser, so I guess I should just accept the risk of tearing. Though I wouldn't be adverse to perhaps inserting a couple centimeter wide band of Lycra down the whole length of the inside leg seam? In matching color of course. Then I could climb stairs without fear.
I like the fit of the jacket a lot. Given the texture and weight of the material, I like how it hangs in a more relaxed manner. It's likely the angle of the photo, but it appears that the pants weren't cut with the same fit in mind, however. They seem a bit slim to my eye and accentuate the "loose" jacket look that forex is referring to. Again, likely just the picture, but I'd prefer a slightly fuller pant to better compliment the drape and look of the material. My $0.02.