Head to your Rewards section via your profile tab (in the top right corner of your screen). If you have a bonus bet that has a split option in your account, select the 'Split Bonus Bet' button.
Step by step...
Head to your Rewards section via your profile tab (in the top right corner of your screen).
If your bonus bets have the option to be split, they will display as below.
Simply click on the 'Split Bonus Bet' button to split your bonus.
Once you have split your bonus, you will not be able to split it any further.