How can I allocate more of our self-employment income to one spouse?

Huh?  Self employment is one person.  One Schedule C.   The self employment tax goes to the person the Schedule C is for.  You can't allocate it.  Are there 2 Schedule C, one for each of you?  

There is no way to manually allocate a Self-Employment tax if you file a joint return with two Schedule C's.  Self-employment tax is determined by the income and expenses each Schedule C reports for each person.