cat: illegal option -- 2 while merging
The first file you want to cat starts with a - so cat considers it to be an option. But as there is no option 2 (see the usage string) it throws an error.

Use one of

cat ./-2e-5_cas 0_cas &gt; prova
cat -- -2e-5_cas 0_cas &gt; prova


instead (the second is better for scripts etc).

PS: In general it is recommend to not use - as the first character of a filename to prevent exactly these issues.