Can I view messages from two different iPhone's on my Mac?

Andrew, as much as I would like to find a way around it I can't seem to make anything work with two different Apple IDs. Two different phone numbers, sure, but two different Apple IDs, no. You would need to "sign out" of one and "sign in" to the other in Messages > Accounts, or log in under two different Mac User accounts. That falls short of what I think you want to accomplish.

Don't take my word for it though; yours would be a good question for Apple Support. Click the Support link below. I'd be interested if they have any better ideas, but I'm nearly certain you will have to be persistent because the first level of support is almost certain to tell you what I just did.

Yes, provided they are both signed in to iMessage using the identical Apple ID.

Let me think about it for a bit. It seems to me there ought to be a way. Give me a few hours and I'll get back to you.