It's just that you'll want to use the gentlest possible cleaning cycle for the completed item. If one is machine wash and the other hand wash, dry flat, you'll want to hand wash and dry flat. You can never go wrong be erring on the side gentleness.
Besides, sometime combining two very different yarns can result in something very lovely and different. Never be afraid to experiment. About the only caution I'd through out is to understand that some yarns are easier to use than others.
I mostly crochet. I know how to knit although I'm a very bad knitter -- can't go more than ten stitches without making a mistake. It's been my observation that absolutely any yarn can be knit, because of the way the stitches are held stable on the one needle while you use the second needle to work the next stitch. So even pretty esoteric yarns, like eyelash yarns or very bumpy novelty yarns, can be knit. When crocheting, the stitch is simply up there in the air while you use the hook to work the stitch. There's nothing holding it in place, except the shape of the stitch itself. As a consequence, a lot of novelty yarns, like eyelash yarns, cannot (at least I cannot manage it) be crocheted unless paired with some kind of simple yarn to give them stability. Then they crochet beautifully.
Took me several years to figure this out, but a lot of the fun of crocheting or knitting is figuring out some different things to do with all of the wonderful yarns out there.
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