New FBM seller question

Just listed my item yesterday and obviously doesn’t have the buy box. The high quantity box is too big to send to FBA but it can be split and made into smaller boxes. I still want to sell the bigger quantity box cause there is a better market for it. So, can I send smaller boxes to FBA to get the buy box and will that help the bigger quantity box have the buy as well? Since both will be on the same listing, just one will be smaller quantity FBA and the bigger one will be FBM. TIA!