Hi, I need to add an interface adapter (pair) to a V5200 running in production. I found the following procedure

Removing and replacing an interface adapter

All hosts have multipathing enabled and access to both cannister. 

Can you confirm that can be done online? Any other things i need to take care to prevent lost of access?

You need to stop one controller at a time and your hosts will continue to access the volumes via 2nd controller. You add/replace your interface card as shown on ibm docs. put it back and wait till it's online. Wait for multipath discovery for 30 minutes after going online and do the same thing on the 2nd controller. 

double check your servers multipath and choose "maintenance window" during low workload. just remind that boot process for each controller takes 20 mins approx. I would set a 2 hour maintenance window for that kind of job.