Good idea, gottaknit (and others)....a soft border may help.
Here's what I swatch-tested last night:
from L to R:
both yarns held tog, garter stitch
garter stitch multi-colored yarn
garter blue only
garter multi again
Then blue only in stockinette
AND: blue alone; and both held tog.
Now I've confused myself, not sure what I want to do...preferences?
Holding both together will make the blankey go faster, but it will be thicker and more rigid. I'm using size 9mm in these pics.