- decide who actually "needs" the benefit
- figure out how to recognise these people ("means-tested" benefits!)
- catch the people defrauding the system
Instead, you could just say, e.g. for the case of child benefit, that every child receives $400/month and get rid of:
- the complexity in deciding who gets $400/month
- the enforcement of fraud
The "underserving" middle- and upper-income will receive this money too, but it doesn't matter - it's effectively coming out of the higher taxes they are paying already.
UBI is just a generalization of this idea to all benefits.
Introducing two kinds of dollars smacks, to me, of the same mindset that introduced means-testing/etc in the first place: "we don't want fraud", "we want this to only go to deserving purposes". Lots of complexity, hard to manage, and for no convincing benefit.