Don't be ridiculous. The memory foot print of Max ( and to a lesser extent Maya ) is huge. And the way that the Ram and VRam is accessed means it's super inefficient. A single App is way more productive.
So I am pretty sure you cannot copy and paste between instances of Max at all. You have to save a what you want and merge it in.
Maya you can copy and paste between instances. But there is a clipboard size limit and it crashes more often than it works on larger scenes.
I've copied 10gb scenes with C4D with no issues at all.
"If Max Crashes" - I like your optimism.
I get that YOU may have a use case for long single threaded tasks.... but that say more about the prehistoric nature of Max's core ( coming from someone that used to love it )