Can we spell a third number?
J, 19 bytes
Takes input as six four. As the input range is limited, I just deleted characters while the output stayed the same. :-) Reads like &quot;if there is no e in the input, or if there are no t and o in the input, … then we can spell another number.&quot;
e.e`to`for`si-.~&amp;&gt;&lt;

Try it online!
e.e`to`for`si-.~&amp;&gt;&lt;
                &amp;&gt;&lt; boxing stuff
  e`to`for`si       for each e, to, for, si:
             -.~      delete from the input any character from the list
e.                  is the input still in the results, i.e. is there any 
                      case were no character got deleted?