He just uses too much air (bevore, he was breething from my tank). He doesnt need the second tank every dive, but it's saver and more relaxed to take it. He uses this configuration since allmost 10 years and I allways check, if it's okay for the company we dive with when we plan to there.
Greetings from snowy Vienna!
How does he carry the two tanks with him underwater? Sidemount or doubles on his back?