Usually this is done with the "delay" measurement on Tek scopes. I
looked through the TDS3000 LabVIEW library and it doesn't look like
this measurement is implemented on the TDS3000 series.
There should be another way to do this though. You will know where the
first signal is based on the trigger threshold. You could then read the
second waveform as an array of doubles. You could iterate through the
array and find the point where it crosses the value of the trigger
threshold.
Alternatively, you could read both waveforms as arrays, iterate through
the voltages of both waveforms until you see the values you want, then
subtract the indices to the arrays and multiply that by the horizontal
resolution.
It looks like the LabVIEW driver implements everything necessary to support both ways of doing this.
