Act 1a says: "Action > The investigators spend 2 clues per investigator, as a group:..."

I interpret this as they do not need to be at the same card location, just one action is spent by one investigator, and there just need to be 4 clues spent total for two investigators, no matter their source. Not two from the two separate investigators. Am I interpreting this correctly?

Just one of the frustrating things, as an experienced tactical gamer, playing this game for the first time: Language has to either matter and be defined in the glossary, or not matter and not be defined, in my opinion. I encounter language often that seems like it should have glossary definitions and doesn't. It is incredibly frustrating getting started here, to figure out the balance between what language should be interpreted absolutely literally and by glossary, and which language is flavor. Game designers are people. I get that. I just feel like a lot of these things could have been anticipated through play-testing and being gracious to one's audience. Just my early reflections.

You interpret it correctly. One investigator has to play this action, than any investigator at whichever location can contribute clues for this. All clues can be from one investigator, three from one and one from the other or both contribute two equal.

In later campaigns, you will see that act sometimes directs you which investigators can spend clues and where.