This is one of those situations where two low-powered controllers from different technological generations can't do what one high-powered controller can.
The 1768-L45 supports only SERCOS motion control over fiber-optic connections, using the 1768-M04SE module. The Ethernet modules available for the 1768-L45 (1768-ENBT) do not support CIP Motion over Ethernet.
The 1769-L33ERM supports only CIP Motion over Ethernet using its built-in Ethernet port. It does not support a 1768-bus connection or any other SERCOS interface.
You could do a Produced/Consumed Tag between these two controllers that gives the position and velocity of an axis, but it might not be fast or accurate enough to actually synchronize the position of two conveyors.
Another possibility might be to use the 2198-ABQE module, which is an Encoder Output module that synchronizes with CIP Motion axes over Ethernet. I don't have direct experience with that module, but it's worth looking into, to see if the servo drive(s) on the 1768-L45 system can accept an encoder input for gearing/tracking to the speed of the other conveyor.