The exisiting outside se cable is 2/2 SE, it feeds 2 gang 100 amp sockets, the exisiting mains on the two panles are 100 amp mains. On the owner occupied unit, he has electric range, electric dryer. 30 amp 2 pole feeed going out to a gargse subpanle. The owner would like to add central a/c to his side. With doing this I believe that he would have to upgrade to a 200 amp service. My question is when I upgrade the service can I install 4/0 alum for a 200 amp service of do I have to use a 300 amp rated feeder( 100/200)? I have found a 200/100 rated socket and beileive this will work as it is approved by utility.
Depends on how you do it an what the load calculation is.
You can have a service conductor supplying two to six disconnecting means that only needs an ampacity equal or greater then the calculated load. But if you have an individual riser or underground feed to each disconnecting means they each need sized to the overcurrent protection devices associated with their respective disconnecting means.
If your load calculation is only 150 amps for both together - you could supply them with a common conductor with only 150 amps ampacity even though the sum of the overcurrent devices is 300 amps and it would be NEC compliant.
Clear as mud?
If changing both meters- I would consider a 200 amp socket for both in most instances - someday the second unit may need upgraded as well. Of course it becomes trickier to size a common feed to them when load is unknown, but maybe use best guesstimate and it may be fine later on. Chances are it will not be any more load on the second unit then what is currently going to be on first so maybe use that figure if allowing for future change to the second unit.