A good way to check this is to knit two swatches and weigh them. That way you can see for yourself how it works,
Do you want the sweaters to end up the same size, or do you want to follow the same size in the pattern and have the garment come out different sizes?
If you want the sweaters to end up the same size, make the swatches measure the same size (approximately). This means casting on a different number of stitches and knitting a different number of rows so that the two different gauges can make the same size swatches.
If you want to knit using the same numbers and have the sweaters turn out “as they will”, then cast on the same number of stitches and knit the same number of rows to make different size swatches.