This depends on several factors like what type of hardware and API you are using.
The PCI 60XXE family only has one ADC so the data can not be simultaneously sampled, but it will sample the channels as quickly as possible. There fore if you are using an E-Series device you just have to include the additional channels into your scan list. The triggering will take care of scanning all of the channels in the scan list and then wait on the next external pulse.
The PCI 61XX (S-Series) has one ADC per channel, so it can perform true simultaneous sampling. Other than the physical hardware it operates the same way as the E-Series device and you just include all the channels you want to scan in the channel list. This is assuming your external pulse is just wired
to your SCANCLK pin you have set up your acquisition for an external clock. It is important to note this clock just starts the scan of the channel list and then waits for the next pulse to scan the channel list again.
You didn't mention what API you are using. In LabVIEW it is very simple. You just change your channel from 0 to 0,1,3,4 or 0:4 or something else similar to that. If you are using direct driver calls like DAQ_Start you will need to change these functions with the SCAN_Start functions. There should be several shipping examples for this case installed on your hard drive.
I hope this helps.
Joshua
