I understand what the op is saying.
Just keep the two parts in the same folder and unextract the first part, it will automatically unextract the 2nd part with the first part into the iso. You don't need to extract the 2nd part ever. Some will even have 3 or 4 parts, but you always just extract the first part only.
Edit: crap I didn't notice you have a mac, nevermind I'm a pc.
"I'd killed more cops than cholesterol" -Max Payne, Max Payne 3
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