You can try using the Calculate Date Time component. Set the date to be the Date on the stream, and set the Time to be the output of the TimePicker component.
Then you can bind the Date Default value and the Time default value to your DateTimePicker component.
Attached a .package to show what I've done (it's a .zip, but just rename it as a .package, and ignore any warnings on open that it's created in a higher version - I'm on a higher build)
If you can't open, I'll describe it:
1st comp: FormBuilder with TimePicker... output is abc
2nd comp: Add New Data Element, adding a date to the stream... output is def
3rd comp: Calculate Date Time... Date value is def, Time Value is abc, output is calculatedDateTime
4th comp: FormBuilder with DateTimePicker... Date Default Value is calculatedDateTime (uncheck Start Today to expose Date Default Value), and Time Default Value is calculatedDateTime
Hope this helps