How to trigger or start two counters

Using a USB6212 M series, I have two quadrature encoders feeding into the two counters. I use the freqout source to provide a 6250Hz clock signal for both, and run continuous sampling of the fifo at 200 samples per  loop. The code, in its most basic form, is attached.

I am having real problems getting any of the DAQmx vis to start the two counters at the same time. The purpose is to measure the relative rotation of the two encoders. Evidence is that one counter starts fairly randomly around 30 counts or 5ms before the other, which ruins my relative rotation measurement.

I'll take any suggestions. I don't care whether the trigger has to be a digital input, or a front panel event or whatever.

Thanks in advance.

Chris