This video shows an Ashtech ProMark 200 receiver fixing with L1L2 and compares the time-to-first fix with L1G1 mode.
Corrections were provided by The Utah Reference Network. L1L2 fixes were 3 to 5 seconds, while L1G1 fixes were 3 minutes.
While L1L2 is clearly superior for RTK time to fix (and robustness of fix when moving or under canopy) L1G1 works with the
internal GPS antenna and has extremely short occupation time requirements when used in a PM100 base rover pair.