Can I use the external trigger to simultaneous capture waveforms on two oscilloscopes?
From this document: "Most Tektronix oscilloscopes also provide a discrete output that delivers the trigger signal to another instrument—a counter, signal source, or the like."

It's probably on the back of the scope, and you may need to read the manual to get it working right.  

If you were operating at 100MHz I'd also caution you that there's probably some timing skew -- but unless the oscillators have much sharper rise times than their frequencies and you're comparing nanoseconds of difference, then you're probably OK.  If you do need to resolve things that fine, get everything working and swap inputs between the two scopes to see how much they change timing.  Ideally you'll be able to calibrate for any timing offsets.