You might find this helpful - a webinar on some Monarch techniques called "Invisible Data" that covers the Recno() and Rowno() functions:https://datawatch.wistia.com/medias/fa64bjflsm
The answer is yes - it always is with Monarch - but you might need a little preparation. If your data was in separate tables, it's easy, a filter defined as mod(recno();50)=1 will pick out the 1st, 51st, 101st etc record from the data.
To combine it and get the results you need you'd need to make sure all the checking accounts were sorted before all the savings accounts, and then have a filter like (type="checking" .and. mod(recno();50)=1) .or. (type="saving" .and. mod(recno();20)=1)
But - and do please play around with the functions and watch the webinar for tips - Recno() is not the same as Rowno() - record numbers are calculated from the data source, before sorting and filtering. If you need to filter and sort the data first to get the checking accounts above the saving accounts, you'll need to define a calculated field based on Rowno() and then export the table, then open it back up again.
Just to close the loop on this one in case anyone every references it, I took Olly's advice and did some playing.
I landed on using a formula based filter Recno()/50=Int(Recno()/50) for every 50th account. Worked like a charm. Little bit of overhead moving files back and forth between Monarch and Data Prep studio, but got it done quickly none the less.