This is pretty neat! It reminds me vaguely of a project I did a long time ago, measuring latency and jitter in MIDI interfaces. I didn’t have an oscilloscope available, though, so I used an audio line-in jack as a cheap two-channel voltage sampler. If you make sure the stuff you want to sample is in the right voltage range for an audio line-in jack so you don’t fry it (or get clipping), and record a minute or so of line-in input, you can do oscilloscope-style analysis offline with the recorded file, e.g. either just view the two channels side by side, or compute statistics about delays between events in one channel and matching events in the other.
Nice! I was thinking of something similar with a USB input interface that takes 2 (or more) inputs – something like a Focusrite 2i2 or such to avoid having to be too careful with output levels.