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Your best bet is to speak to sales to negotiate any deals.
Thanks Jeneral Pain
I was intending to chat to them shortly (if I can get that phone from my daughters hands) but having a bit knowledge in advance is always handy
There's nothing specific to multiple handset discounts at Optus. Sales staff also seem to be a bit limited to offering whatever the deal of the month might be (although never any harm asking what they can do for you personally). In general though I find the biggest discount of mutiple phones is the family shares the data.
As it happens you might be in luck as Optus just started one of their 48 hour flash sales.
$40 a month gets you another 80Gb to your current plans to share which is about as cheap as data gets at the moment. Also includes 4Gb for overseas roaming a month in case your daughter likes to travel.