In reply to AliRLee:
If the diameter of the static and dynamic ropes is similar it'll be totally fine. The stretch difference won't be very noticeable unless you have an unusually static static and a very stretchy dynamic. Can buy a slightly longer static than dynamic if you're worried.
If you're wanting to use a skinny static cord (e.g. Petzl Pur Line, Edelrid Rap Line, just a bit of normal static accessory cord) then you'll want to be more careful about abseiling normally on them. Probably you'll end up using the skinny static as a pull line/tag line. This is more annoying than normal abbing with two ropes, but works fine and isn't loads slower. There's lots written about it elsewhere on the internet. Worth making your static pull/tag line 5-10% longer than your dynamic rope, it won't be being weighted on the abs, so won't be stretched atall, whereas the dynamic will stretch 5-10% ish.
Post edited at 15:52