Now, about plying mixed yarns from singles that have been previously set. I have been plying cotton with flax (or is it linen at this point?) and then dyeing it. It really makes a pretty yarn for weaving. In my first experiments I boiled the plied yarn - and I think I'll continue doing this for this project. But, I was planning, in general, to finish the singles before plying for mixed yarns. My concern here is that the twist will already be set and I'll have difficulty achieving a balanced ply. Should I be concerned about this?
Both cotton and linen can take the high heat of simmering. If you simmer and set the twist ahead of time, on both singles, you will need to experiment a bit more to find the correct amount of plying when you put the two parts together. You can ply until the yarns look plump and full, but the plied yarn will be very twisty.
However, after a soak, to thoroughly wet both plys the original twist will be revived. Neither of these fibers has very much "body" or resistance to being twisted. So it is wise to put a fairly firm twist on each as a single. It is also very helpful to save a short length of each single plied back on itself. The correct plying twist will be very much like the way they each plied back on the short samples.
I checked by plying together some very old singles, (15 years, I do save everything, big sigh!) that I had on hand, of cotton and linen, both had been simmered previously to set the twist as singles and I had the original self plied samples. After plying the yarn was very twisty and looked incredibly overplied. After soaking and washing, (reviving the singles twists) it makes a nice plied yarn.