Re: Should couples invest in Bitcoin together or keep separate portfolios?
This makes sense. You get to have exchange wallets, one name but two owners and both of you can access it. While no one can stop both of you from owning separate wallets for as many as you can. That's fair enough and I think partners are going to agree with that idea and it's better with that way while both have their passes if ever they wanna go ahead and exchange some.
Hmmm, that looks awkward!!! 
Why Exchange wallet, though? Did you forget about it, "Not your keys, not your Bitcoin". If their parents have intentions to have a joint wallet to keep Bitcoin for longe term then they need to create a multi-sig wallet because it will allow both of them to have access to their wallet as well as no one to be able to withdraw funds without the consent of others. It's also safer because through this they will be once own the key and phrases of the wallet.
Besides that, for personal investment, everyone has their own preference for whether they want to go for an exchange wallet or for a non-custodial wallet. It's is totally depends on your act, like it is up to you. But for a joint wallet, it would be better to go for a non-custodial multi-sig wallet instead of an exchange. So it will keep their funds safer...