It isn't uncommon on a recumbent trike, where balance isn't an issue with really low gears, I haven't seen it on a convectional bike, but there's no reason it wouldn't work. I've used a double on an Alfine 8 speed, without problem, not really very different to using a cassette. The Rohloff tensioner has a capacity for 10 chain links, which roughly translates to a 20T chaining difference*, you don't have to use the Rohloff one, though most others don't offer an acceptable chainline without modification. You'd have to play around with a gear calculator to decide where to put the extra gears, I think realistically it's going to provide an extra three. On the Alfine I set the extra gears at the bottom, it was a mistake, shifting at the top where you can afford to lose some momentum would have been better. Chainline isn't going to be critical, setting it for either of the chainrings or between them isn't going to make much difference. You still have the advantage of the chain only running to one sprocket, lots of ways to shift, I'd choose 8spd, friction shifting and a jump Stop. You might also consider the Rohloff's maximum ratios, it's up to you whether you treat them as a rule or a guide and that might depend on whether the hub is still in warranty.
I occasionally spin out on a Rholoff, but not often enough to consider doing anything about it, the bottom gear is a low as I can go without falling off and the top gear is high enough that even with more gears it would be a lot of extra effort for marginal gain. Nor saying someone else might not get the benefit, but the numbers need a proper look at first.
* EDIT - No need to do the sums, there's a choice of chain length calculators, like this one:
https://www.alpinetrek.co.uk/chain-length-calculator/
