multiple waveforms with different start triggers

I have four waveforms, AO0, AO1, AO2, AO3 - I want to have each waveform start with a different start trigger, PFI3, PFI4, PFI5, PFI6 respectively.  When I attempt it, it says the clock resource is already in use.  How do I share the clock for the different waveforms?

Can this be done on the NI-6722?

Thanks

Bill