As an aside: mIRC is a single threaded software, so if you are going to run two instances with any significant scripts or large number of channels, you can manually assign the "Processor Affinity" of one of the instances, so it utilizes a secondary core/CPU. Otherwise both instances will utilize the same 1st core.
(This is why Process Manager says mIRC maxes out at 50% CPU on a dual core, 25% on a quad core, or 12% on an 8 core)