Two different domains, two different configs, same machine

[Nicolás]

Hi,

Up until now I've been using Postfix for one domain, evidently with just 
one configuration, using the virtual domains approach. Now I'm in a 
situation where I need to handle another completely different domain, 
with a different behavior and a different Postfix configuration, on the 
same server. My question is: Should I use the Multi Instance approach 
(http://www.postfix.org/MULTI_INSTANCE_README.html) in this case?
I'm asking because I'd need to use the same server public IP for both 
instances, and I don't know whether Postfix would create two different 
processes (one per each instance), which would mean I need at least a 
different port (and I even found several examples where each 
configuration has its own IP address), or if both configurations are 
handled by the same process.
If the Multi Instance approach is not suitable for this scenario, would 
someone recommend another way to face it?
Thanks,

Nicolás