Yes, yes, I know that this needs some fleshing out, but I'm figured that it would be worth putting the basics up here as a checklist first, before I forget the process.
Status: two people, each with their own iPhones/iPod touches/ipads, each using their own apple accounts, each using their own computers.
Desired outcome: only having to buy music/apps/etc once
How: create third apple account to use itunes home sharing.
a: user one authorises their apple account on user two's iTunes
b: user two authorises their apple account on user one's iTunes
c: create a third apple account
d: setup home sharing on user one's iTunes, logging in as the new, third account
e: setup home sharing on user two's iTunes, logging in as the new, third account
f: you should now be able to see each other's itunes library
There are more detailed settings available, to tell itunes to automatically sync new items, and there is a high chance that this is actually a way over the top, ridiculously convoluted method, and I'm almost certain that I'll discover that there is no need whatsoever to create the third account, but I did it anyway, as I never quite trust itunes to not get crazy confused when logging in like that on a different computer.