It depends on the situation or personalities.
What if both of them are knowledgeable in crypto but have a different opinion on alt coins, or what if they are in a verge of breaking up?
It is good to have a partner with a similar mindset or have the same interest as you, you could both have your own investment or portfolio or just combine your investment to make it grow.
If they are both knowledgeable in crypto and have their own preferences, they can choose to invest differently, but even as that, they can still have separate investments and a combined one. Having knowledge of crypto can even make it easier, as they can use a wallet that requires both their permissions to make any withdrawal. Even in terms of going their separate ways, everything is based on agreement; they can split it.
Two good heads they say are far better than one, if the both have the same interest, doing it together can enhance the investment in terms of finance, time or attention given to the investment. However, there are couples who might not want to have a share of the same investment we still have to respect their own opinion on standing on their own rather than having a joint investment, if they can invest together is ok and if they can not it is still ok provided it is their personal decision and choice.