You can split the $30,240 between traditional employee elective deferrals and employee Roth contributions (technically not a deferral of taxable income despite colloquially being called Roth elective deferrals as is done in TurboTax) any way you want. If you make at least $8,000 of these to the Roth account, you'll have at least $8,000 of compensation available to support a Roth IRA contribution. So yes, you can make the split that you propose.
