Ive made three really ugly strings consisting of four/4 strands of fast flight plus for the body of the string and twelve added strands of b50 in the loops. They worked, they were durrable enough to be dragged through the woods for dozens of hours over the course of a month, but they were noisy and took a lot of twist to adjust.
But anyway, I wouldn't suggest mixing material. Ff can take being the only thing working, but the other stuff wont be taught under tension without a bunch of fenangling.