Iterating over two lists in parallel in /bin/sh

I have two lists of equal length, with no spaces in the individual items:

list1="a b c"
list2="1 2 3"

I want to iterate over these two lists in parallel, pairing a with 1, b with 2, etc.:

a 1
b 2
c 3

I'm attempting to support modern portable Bourne shell, so Bash/ksh arrays aren't available. Shelling out to awk would be acceptable in a pinch, but I'd rather keep this in pure sh if possible.

Thank you for any pointers you can provide!