Increasing yarn weight

[hydrolagus.livejournal.com]

I'm going to knit Calorimetry for a friend who really liked mine. She loves blue and sparkly. I have some thin but pretty blue--too lightweight for Calorimetry by itself--from a sweater I frogged and I could get some sparkly. To use them together, would I just draw from both balls at the same time, or do something like spin them together first? Or should I stop being silly and just buy some blue sparkly yarn that's already an appropriate weight?