Never used anything quite like this before

Dual Flowmeter on HTP’s site:

https://www.usaweld.com/Dual-Flowmeter-with-Backpurge-p/dualflowmeter.htm

HTP Dual Flowmeter on Ebay:

https://www.ebay.com/itm/171091787598?ru=http%3A%2F%2Fwww.ebay.com%2Fsch%2Fi.html%3F_from%3DR40%26_sacat%3D0%26_nkw%3D171091787598%26_rdc%3D1

A forum thread on HTP Flowmeters:

https://weldingweb.com/showthread.php?t=272541