I came across the word twindemic the other day. Yes, it was quite a shock.
A twindemic describes two epidemics happening simultaneously: as, for instance, 'flu and Covid-19.
I do not wish to think about that even as a possibility, but if it happened there'd be not much twinnish about it. 'Flu and Covid-19 are rather different diseases.
Calling it a didemic would make more sense - if you know any Greek it would, anyway - except that in that case you'd also know about the derivation of the -demic bit and then the whole word falls to pieces anyway.
In any case, are we having a singletondemic at the moment?
Of course not.
Word Not To Use Today: twindemic. This word is a mash-up of twin and epidemic. Epidemic comes from the Greek epidēmis, which means among the people. the demia bit meaning people. The Old English form of twin is twinn.
The Old Norse tvinnr means double, but that's still no excuse.