You would have to use a catalog client script on variable set 1 to achieve this. The script would need to be an onchange script that triggers most likely upon a row being added to variable set 1. This would depend on what/when you want the data populated in variable set 2.
Depending on if you want Variable Set 1 and 2 to match exactly or not the approach would change.
In either case you would have an onchange catalog client script applied to Variable Set 1 that would trigger on row add/delete/update depending on how you want the two mrvs to be related.
If you want them to be identical then you would have the script retrieve the value of mrvs and then use a g_form.setValue(mrvs_internal_name, mrvs_1_value). This works with very little effort if the two contain exactly the same variables and variable names. If they have different variable names or they aren't simply identical then the approach changes a bit and you will need to manipulate the json array of objects between when you retrieve it from mrvs 1 and apply it to mrvs 2.
Be careful because the g_form.setValue() method sets the entire mrvs so using it will erase whatever was in it before. So if you need to add a row to mrvs 2 when a row is updated in mrvs 1, similar to creating a change log for mrvs 1 then you will need to retrieve the value of mrvs 2, append the new row to it in script, and then us g_form.setValue() to update mrvs 2 to include the new row.
