If you can pay in less than 6 months, asking for an installment plan is kind of pointless. You pay the same interest and penalties for late payment as if you just paid late and waited for a bill for interest. Also, requesting a payment plan in Turbotax may result in a non-refundable application fee that you would avoid by applying directly on the IRS web site.
In Turbotax, select that you will "pay by mail". This way, Turbotax will not make an automatic payment for you. Then, go to www.irs.gov/payments and make the payment you desire. When you have the rest of the funds, make the other payment. You may be charged about 1% per month combined interest and penalties on the amount not paid by April 15.
If you can't pay in 6 months, you can request an installment plan online. You will still pay interest on the unpaid balance, but the penalty for late payment is reduced.
