Some crochet patterns will tell you to work with two strands of yarn at the same time. Some projects require you to double up or use more yarn strands at one time for thickening up a project. Web here are some additional ideas for keeping your yarns separate:
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Web learn how to join yarn together without ending up with any ends to weave in with this video crochet tutorial, courtesy of chris hammond of allfreecrochet.
If you want to use the same.
You see this commonly used.Placing each ball of yarn in its own bowl helps to keep the yarns separate and is especially helpful. Work from both ends of the same skein.