You can't split payments on TurboTax, but you can on the IRS Payments site. 

• What you will need to do in the program is select "I will mail a check" as your form of payment, then File your return.
• Then visit http://www.irs.gov/Payments and select "Pay by Card".  
• From there you will be given the option to choose one of the IRS approved payment sites.  

The two we use most often are "Pay1040" and "Official Payments, but any of the ones listed should allow you to split the payment.