I have two male birds ~ 10 months old, and I'm considering whether to get them a tamed-female as a potential mate... but as there are already two of them... is it just likely to cause trouble between them (have only just finally found the perfect rearrangement of their cage so they don't fight over where they sleep!)?

Why not get 2 females and make even numbers?

If you've got two males I see no reason in getting a female. The males can be each other's "mates" (in the friendly sense of the word at least) and adding a third budgie might just create an outcast. Two are likely to bond leaving one all alone.

 

Keep in mind that sometimes budgies just don't get along and you might end up having to cage them all separately! That's happened to me and believe me, it's a pain!

 

It sounds like your two are just fine, plus they have more room in their cage if you don't add more! And if you want to add more budgies, make sure you have spare cages for quarantine or even permanently in case things don't work out.