"Do you agree or disagree?"
Translation:Wyt ti'n cytuno neu yn anghytuno?
Keeping the second yn feels better, somehow. And neu'n instead of neu yn.
A quick skim through 'Pantglas', a novel by Mihangel Morgan, shows that he repeats the yn in this situation, as well as after a/ac, both in his narrative and in the colloquial speech of his characters. T Rowland Hughes in 'O Law I Law', too.
I've looked to see if I can find anything in Gareth King, and the only example I can find is "Ellwch chi aros fan hyn neu ddod 'da ni". So you would make that "neu yn dod" or is this somehow different? I'll be listening for it over the next few days to see what people round here say :-)
There are quite a few of this kind, but I would not like to say that it is either especially common or uncommon. It is juys one of several types o mutaition. You will see a number of other examples if you just browse ang-/angh-/ann-/anh-/am-/amh-/etc in an on-line dictionary - as a negating prefix, a-/an- usually causes a nasal mutation of whatever it is prefixed to.