Yarn Substitution
2 strands of Resolute held double equals 200m/100g, which is a Worsted weight yarn equivalent. 1 strand of Resolute held double with 1 strand of Steadfast is 147m/100g or Aran weight yarn equivalent. 2 strands of Steadfast held double throughout will be 116m/100g, also recognised as a chunky weight yarn equivalent.
Susanna Winter has written an insanely clever article about the maths of yarn substitution when holding double. I strongly urge you to check it out here.
The principles are:
Figure out how many skeins of the thicker (or lower-meterage) yarn it takes to cover the same distance as the thinner (or higher-meterage) yarn.
Calculate the combined weight of the two yarns over the meterage of the longest yarn.
Divide the longest length by the combined number of skeins it would take to cover the distance.
You should now have your meterage m/100g which indicates the yarn weight of your 2 strands held together.