I have a requirement to merge a large number of groupings of two lists
(each an Excel worksheet) with differing numbers of rows, at the ratio
of their original sizes (number of rows).

This is always a fixed ratio, but most often at other than a 1:1 ratio.

For example, 200 rows in Sheet A and 100 rows in Sheet B, merged into
Sheet C with 300 rows, merged at the 2:1 ratio A:B, as 2 rows from A,
one row from B.

How would I do that in Excel?

Thanks.