Applying part of a contribution to a Participant Payment
I would suggest 


Create participant for Participant A
Create participant for Participant B
Create a $200 contribution against J.Smith contact 
Add line $100 line item for Participant A and Participant B with contribution id from #3
Add participant payment entry linking contribution from #3 with participant id from #1 and #2.


If you are using drupal than whole process can be done using webform without any customization. (Probably caldera form for wordpress)

Thanks
Pradeep