That combination is not listed in its manual page. However, you could filter the result using awk/sed/perl (your choice). For the given example, sed suffices:
xxd -b some_text | sed -e 's/^[^:]*:[[:space:]][[:space:]]*//' -e 's/[[:space:]][[:space:]]*.\{6,6\}$//'
That is:
In the first "-e" option, remove everything through the first ":", followed by whitespace.
In the second "-e" option, starting with at least one space
- followed by 6 characters (it's unclear if those can be spaces, but testing seems to confirm they are not)
- strip off the entire matched string
You could replace the "[[:space:]][[:space:]]*" with exactly the number of spaces which you want to match, but though more verbose, the character class is also more powerful.
Alternatively (since whitespace is a default field separator in awk), one could pipe through awk:
xxd -b some_text | awk '{ printf "%s %s %s %s %s %s\n", $2, $3, $4, $5, $6, $7 }'
though as a rule I use the simplest tool which works.