How can i connect two parallel UPS that sharing the same load?

I have a critical load .I want to connect two parallel UPS that sharing the same output, how can i control them??

it's hard and danger to turn on both of them and synchronizing them. i want a controller name or a way to control them.

the controller must turn on one of them in a time and switching if any interutpion in the second UPS occured without disconnicting the load.

Am I understanding correctly that you are wanting to protect against UPS failure?

You're thinking about it the wrong way. The way you protect against this kind of failure is your load must have redundant power supplies, which allows one supply to safely fail when the UPS that one is connected to fails. If your load does not have this capability then hopefully you can have two of whatever it is in some kind of highly available cluster/configuration to allow one unit to fail completely.