What sort of loads are you lifting ? Anything safety related or overhead ?
No
How closely are the drive axes coupled; you refer to them as "conveyors"; is the horizontal axis lifted by the vertical axis ?
axes are not coupled. horizontal conveyor transfer items to vertical conveyor.
How closely do they have to synchronized to one another ?
both should be synchronized in certain length.
If a servo and its shaft brake failed, how would your system prevent the load from falling ?
the motors are attached to gear reducer.
Do the drives have to connect or integrate with a particular type of PLC controller ?