Sometimes at checkout a bidder will want to split a bid with a number of other bidders.
You can do this through the Split Bid page.
On the checkout page select the bidder as normal and then click on View Transactions
Click on the split icon next to the bid you want to split.
See Split Bid for more details on the splitting process.
After splitting the bid click on the Checkout button to return to the checkout page and the same bidders should be selected.