Thickness of yarn
If I am knitting a simple scarf, can I use two strands of thinner alpaca yarn vs one strand of thicker alpaca yarn?
I don't see any reason why you couldn't.
Depending on the relative thicknesses of the two yarns, your scarf will not match the size specified in your pattern, (it will be narrower or wider because of the difference in thickness) but it shouldn't matter than much for a scarf.
If you want to get the width the same as your pattern, you could do a tension square
with the double yarn and adjust the number of stitches you cast on.