Okay. I worked for a natural dyer for one year and she said linen has a slightly waxy coating on the fiber, which makes it resistant to dye [with home dyeing it can be a little uneven or mottled, though not overly so].
First, weigh them when they are dry. If you don't have a scale, put them in a plastic bag and take them to the supermarket and discretely weigh them. One package of Rit will dye 3 dry weight pounds of fiber a medium tone. Rit works on cellulose fibers which linen is, as well as protein fibers such as wool or silk. Of course Rit is not a natural dye, but I'm not sure how the viscose is going to take the dye, as each fiber uptakes differently, and why go to the trouble of tracking down a lot of natural dyestuff for and unknown outcome. I like natural dyes a lot, but I have an old supply of Rit which I'd rather get rid of by fixing it onto material, than putting it down the sewer or in the landfill.
Rit dye is called a union dye; it is all purpose and is composed of dyes which will affix to protein fibers and dyes which will affix to cellulose [plant] fibers. So it won't exhaust, that is become nearly clear when the dye is all taken up. Because the fiber you are NOT using isn't present to take it up. In your case you have no protein fibers to take up the part of the dye meant for them. I am not sure how the viscose will dye; my guess is it might dye better than the linen, but that will just make for an interesting effect. Be brave, bold, and creative. It's just curtains.
Use the largest pot you can find or use your washing machine and pour in really hot water. Be careful. The reason you want lots of water is to have it flow in and through the material evenly [ the opposite of tie dye which scrunches material for deliberate uneven uptake].
There is one more thing. If you have more than 3 pounds you will have to decide whether to have a lighter shade, or get two boxes . Don't breathe in the dust. DO dye all the fabric at one time so they match, and come out the same shade.
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