A seemingly fairly rare motif is that of notes with heads on BOTH sides. The "head" side is almost always the obverse, with the reverse being reserved for other, non-portrait artwork. This thread is for basically, notes that seem like either side could be the "obverse" side, since both sides have similar portrait heads. First example is the Australian 100 dollar. The only way I can tell that the Lady side is the obverse is that the signatures are on that side. The Gentleman side could easily be the obverse too.
I have a Russian hundred rubles from 1910 with two faces- Catherine the great, and possibly Apollo? I wasn't able to find my original note- I hid it in a drawer, so I found this Internet image just to show an example. Other than that, I have no additional notes with two people/heads.
All current notes issued by the Bank of England feature two (or rather 2+) portraits. One is always Queen Elizabeth, on the other side you have ... £5: Winston Churchill £10 Jane Austen £20: Adam Smith (future polymer note: William "JMW" Turner) £50: Matthew Boulton and James Watt Christian
Here's a 1916 Austria-Hungary 1 Krone/Korona. Since Austria-Hungary was composed of Austria and Hungary, the notes of the kingdom had both German and Hungarian text. This one has not two, but three heads.