does he pay dual income taxes?
He does not pay dual income taxes. We live in the US and Britain does not require non-resident citizens to pay income tax unless the income is earned in the UK.
If we were to have income earned in the UK (say, from an investment there) then it gets a little complicated. That happened only once since we’ve been married, and it’s been some years ago. But the income tax paid to the one gets credited by the other. In the US, you use IRS Form 1116 to calculate the “Foreign Tax Credit”. You only pay the difference, if there is one.