Okay let me see if I can do this with maths:
Person A gets paid 10 an hour with overtime paid at 12. Overtime is easily and readily available.
Person B can only find a job which pays 9 an hour.
Their options are:
Person A works a 40 hour week and then 15 hours overtime. This works out to around (10x40 = 400, 12x15= 180) 580 gross per week. They pay for one lot of travel, and one lot of work clothes. There is one commute.
Person A works a 40 hour week. Person B works a 15 hour week. This works out to around (10x40 = 400, 15x9 = 135) 535 gross per week although this is raised slightly in take home due to tax allowances. However there are now two lots of travel to pay for, two lots of work clothes and you lose an extra 1-2 hours per day due to tge second commute.
Both couple still need UC because wages are low.
£9 an hour is less than NMW isn't it?
Regardless, you're missing the point. If it would be preferable for the person working already to do overtime rather than the other person work part time, why are they not doing this already, before this rule change? Their income would be higher and they'd need to claim less in benefits. They have therefore been deliberately choosing to work at less than capacity and have others top up their income more than is necessary.