If you hire good installers they will make the patterns line up. Usually involves cutting and sewing pieces, although sometimes it can be done by folding if the carpet is super pricey, like handmade, (and will be taken with you when you move).

I'd recommend having an installer measure and order baseed on that.

We had an "oriental" wool runner installed in our old house (2 90 degree turns in the stairs) and it's beautiful - I think the install cost more than the carpet thoough.

Good luck.

go to a reputable high end floor coverings retailer and discuss this with them. Especially ones that specialize in wovens and sewing.

as long as there is a lengthwise pattern match in the runner, almmost any runner pattern can be made to line up but this can requires alot of angle cutting, seaming/sewing and pattern dropping because of the stair angles and radius'es, which end up consuming exponentially more runner material, and the larger the pattern match the more material is consumed to line up the patterns, because of waste factor in making the patterns line up.

Expect alot of seaming or sewing.

Expect to pay top dollar ... probably more than what you figure as top dollar.

Most people specializing in this type of work will want or require you to purchase their materials.

It's all custom work.