Hi op.
I have experience of a tfmr at 22 weeks. Out dd2 had trisomy 18 which is lethal.
I had an amnio at 21 weeks after dodgy bloods and some soft markers on our anomoly scans.
The 1 in 100 mc rate that is quoted by the nhs is actually quite inaccurate. It is a figure based on a very old report which itself was based on a collection or small studies. The doc who did my amnio had never had a healthy baby die after an amnio. The figure also included those babies who were very ill and died between the amnio and the results, those babies would have died anyway, regardless of having an amnio.
You can have a harmony or panorama test, I've had the harmony. They are brilliant for a clear negative screen but are only 99% accurate for Down's syndrome and less so for the other trisomies, many people would be unwilling to terminate with those stats. An amnio is 100% diagnostic.
I think there are some additional risks of tfmr with twins (would be termed a selective reduction) so if I recall rightly there is a window at which is is best to do.
I'm so sorry you are going through this, especially with your DH and you disagreeing.
You could come at it at a different angle. I decided an amnio was worth it as I could make what I thought was the right choice for my baby. In terms of twins I'm not 100% sure but the risk of one dying inutero put the other at risk (I think, there is a lovely lady on here who has had a selective reduction and went over all the risks clearly)
I would echo the pp suggestion of arc, they are a brilliant, impartial but well informed charity who can go over all the risks with you.