It's not possible to to get a virus that is a combination of the flu and a coronavirus, because they are very different. For example all coronavirus are "positive sense" RNA virus and the flu is a "negative sense" RNA virus.
https://en.wikipedia.org/wiki/Sense_(molecular_biology)#RNA_...
(With a "positive sense" virus, the cell use the information in the virus to produce proteins. With a "negative sense" virus, it's necessary to make an inverted version of the RNA and then produce proteins.)
You probably read about getting both illness at the same time, not a single virus that is a mix of them.
Also, there are some test to make a vaccine against both flu and covid-19, but it's just a mix of the two vaccines. (The combination must be tested in case it has some nasty surprising side effects, or the body only makes protection against one illness, and other possible problems.)