Having done many, many thousands of miles on long journeys in a Mini, I can assure you that the 7½ gallon tank will take you further than the human bladder will.

Stopping for food or drink or emptying food or drink will always happen more frequently than filling the car with fuel.

No it's dual SU carbs.  Probably won't try to install another tank as I drive pretty much local and at that it get good mileage.

Is the car an SPI?

If it is fitting a second tank is not straight forward.

Being an injection model it will have an in tank fuel pump which has all the pipe work feeding in from the top. Carbed cars use tanks that gravity feed so all the fittings are at the bottom. There is no off the shelf solution to plumb the two different types of tank/delivery system together.

Well the truth is I've always  liked Mini's since I started autoxing and way back then they were awesome.  Sooo, when I retired I just had to have one.  Found mine in SFO, drove it home to Oregon and with Mini-Mania and Jeremy at Jet Motors in Portland it's been a great ride.

Whoah!
I missed that. I thought he was referring to BINIs in general, not your photo of your Elf's boot.

Watch your language. It's not a Bini, it's an Elf. Exactly the same boot dimensions and tank position as a regular Mini, other than the few extra inches at the rear. 

Fewer smiles to the mile though.

Lot more room in a Bini than in a Mini.

Twin tanks aren't that hard to install. You just need to put in the mounting points and strap and get the twin tank crossover t-pipe. I had twin tanks but then I upgraded to the family size 15 gallon option. 

Thanks Dan I should have figured it was Imperial gallons when I put 7.9 gallons in it the other day.

Our host shows a right hand tank.
Classic Mini Cooper Fuel Tanks (minimania.com)

Your fuel tank should be 7.5 imperial gallons. (Not sue about fuel injected Minis.)  At 1.2 US to Imperial equals 9 US gallons.

Right hand fuel tanks as fitted to Cooper S models are scarcer than Hen's teeth. (Left hand tanks won't fit in the right side. An after-market product was a dummy filler to install on the right rear corner. If that is what you have, there is probably not even a hole in the body behind it for a true filler neck. You could install an after-market tank in the boot but that would affect access to the spare tire and/or battery. If you have a carb car, it should not be difficult to connect two tanks in parallel, though if your car has a return line, that would be more complex.

The question is: why do you think you need extra fuel capacity? The stock tank should give you about the same range as most other cars.

I have a 93 Mini.  It has two gas caps but only one tank.  #1 it appears to have 9 or 10 gallons even though the owners handbook says 7?  Would it be much of a job to install a second tank?