Realistically, I think its much better to space them out. I don't have experience at Macquarie with People and Planet units, but my previous university I took all the core classes at once. Needless to say, it was an utter chore to go to university and my marks suffered. So maybe its best to space them out a bit, maybe one per year or something like that- if you do eight units, only one of them has to be People / Planet per year.
I get MQ's intentions in creating the criteria to do a 'People' and 'Planet' unit. I agree that it's important to have graduates who can assess the impact something might have on society and the physical world, and can approach problems in multiple ways. So I don't hate the fact I'll have to do them, but I won't lie it is frustrating. Still, lots of universities have methods (UNSW has "general education"), so its not suprising.
Note that I've been told Arts students may struggle with Planet, and Science / Engineering students may struggle with People (that doesn't mean its hopeless, or that the units can't be completed). Your first session at university should be dedicated to (and something I wish I did at Notre Dame when I was there) getting used to the university, the degree structure and your requirements. There is no shame in dedicating time to make things a bit easier and smooth for you, especially in your first semester. So doing both Planet and People in your first session doesn't seem like a good idea. Instead, pick one, and do the next one in a future session / year.
Also- subjects aren't offered each semester, and there is a maximum of 4. The two subjects that may be requirements for your degree will have to be delayed, and they aren't all offered in the summer session. Just something to think about.
Take advantage of Macquarie's flexible structure- I am going to. I am certain it will defuse some stress, and allow me to manage my time effectively. I'm sure there are people who manage to do both People and Planet in the first semester, but I just want to make it clear that you don't have to do that.