The power conversion always will have a huge loss. It is more loss in converting 230V AC to 5V DC, but also the process 12V DC to 5V DC will produce some unuseable heat.

Why I am writing this? Because sometimes it makes more sense to use one DC/DC converter with two USB outlets, rather than two of them.
Just make sure all consumer won’t need more than the provided amount of A(mpere). And this means peak/under load, not when you take one value with your ampere-meter during an idle process.