I have a measurement that needs 4 scope channels simultaneously, but I only have two identical 2 channel oscilloscopes. Is there a way that I can still do my measurement accurately?
There are never enough scope channels, learn to be creative. If you have 4 repetative signals ABCD, take 3 measurements, keeping one channel the same for a reference. AB, AC, AD.
You can also use the external trigger input of the second scope. But, there will be a slight delay that may or may not be significant to you.
It really depends on the definition of "accurately". If your measurements need to be precise (within the specification of your oscilloscopes) for an arbitrary trigger voltage level, signal shape, ambient temperature, etc. you'd better use a 4-channel scope, or accept that you cannot make an accurate measurement.
If you do your measurements with two scopes, you will only be able to calibrate the trigger delay for a couple of trigger levels. Make sure you do this after the scopes had the time to warm up. Nevertheless, as you proceed with your measurements, the delay in each scope will drift as the trigger signal shape changes, or you tune the trigger voltage level, and the precision will inevitably get worse.