Not an expert just got some experience. I assume by tandem lifts you mean two overhead/gantry cranes working together on the same rails. ITIS is a big subject so I have just posted some topics for consideration.

If you are moving standard components then you can develop a generic lifting plan otherwise you will have to produce one for each lift.

This may sound obvious but you need to ensure the cranes do not hit each other. It does happen and can knock one or both off the rails and/or cause the load to drop. Consider installing some sort of collision warning or use a banksman and “locking off” one of the cranes when it is not in use.
The load has to be balanced to ensure the SWL is not exceeded for one of the cranes. Slinging of the load to ensure it does not slip or drop. Try and have lifting hooks/holes build into anything you are lifting, it is 1000% easier. You may have to identify lifting points of your tools.

Ensure there is a clear path for the load and physically check it/barrier off if it is a busy warehouse for things like forklift trucks. Lift the load as low as possible as this will reduce the impact load if anything did go wrong. All chains/ropes should be vertical as it is easy to get into a situation where one crane is pulling on the other through the load.

Some of these cranes can trundle along at some speed, so set a maximum speed which will also stop the load from swinging.

Ensure there is good communication between operators and others taking into account any back ground noise. Consider a permit system if the lift is in a busy warehouse. Try and have no working underneath.

Practice moving the cranes in unison without a load, it does take a few goes. Consider using a slinger/banksman to oversee the lift.

Do not forget about the statutory checks, not only on the crane but the lifting gear. Read the manufactures instructions and attend the commissioning.

The more I think about this question the more I get an uneasy feeling. I have first hand knowledge of a failure when 2 O/H travelling cranes are used together. They are far better suited to working alone and one must ask how on can satisfy the need to avoid collision by practicable mean. A system of work does not sit easily and I have known such systems fail catastrophically. Certainly any fail will require a full engineering re examination.

Walker

The problem for me is that I do not regard tandem lifts with O/H gantry cranes on rails as common place. The place I saw this last was in a plating works where the Scotch blocks were removed to allow travel by both cranes along the track without any restraint thus allowing tandem lifts of parts into the acid vats.. A runaway on one occasion stretched the power feed cable which then exposed a live conductor that went unnoticed. 2 months later a fatal electrocution occurred when a further runaway took place.

Background:

http://www.citb.co.uk/tr...-lifting-operations-a61/