Child benefit for example combines couples income and caps for means testing at £60,000, but doesn’t differentiate when dealing with single parents - so effectively the cap is £30,000 per parent when treated as a couple.
Yes, and the couple are paying two lots of tax, NI and commuting costs on top of the childcare that the single earner of £60,000 doesn't have to pay (assuming that s/he is part of a couple with children). The family with a £60k single earner pays in less than half of the dual-earner family on £30k each, but takes out twice as much.
It's worse again (fairness-wise) when the single-earner family earns just under the threshold to claim money for things like student grants, but the dual income family earn (between them) just over. Again, two lots of effort, two lots of tax/NI plus childcare, but the single earner still comes out better off.