Unlike a mortgage on a house with 2 parties, an Optus account is against a single person on the bill.
You can have another person added to your account so they can call up and get support, but not on the bill.
As @Paddylee says you can add people to provide access to each account.
It may also be worth taking over your partners mobile phone account completely. There's a simple online process to transfer it from them to you.
This has two advantages - You get one bill for both phones. But you also get to share the data you each have for the month.
It also has one potential significant downside - You are the one on the hook for the phone costs. If your partner goes rogue they can leave you paying out the handset costs etc. and can even rack up big bills going over usage allowances etc. It sounds like you're not in that space at the moment but its something to keep in mind.
Oh this sounds like a good option. They outright own a handset so it would just be the bill and if I can find a no lock in I could cancel it if anything turns south (I have been in one of those before) in the relationship! Thank you for your suggestion!
You can request data be turned off (and back on) anytime or cancel SIMs etc. anytime (same as you would if you had to report a phone lost or stolen).
The process should be pretty easy to do here online