I know this much:
$ command 2>> error
$ command 1>> output
Just add them in one line
command 2>> error 1>> output
However, note that
>> is for appending if the file already has data. Whereas,
> will overwrite any existing data in the file.
command 2> error 1> output if you do not want to append.
Just for completion's sake, you can write
1> as just
> since the default file descriptor is the output. so
> is the same thing.
command 2> error 1> output becomes,
command 2> error > output