The ownership of joint accounts is shared 50/50 between the 2 investors. Hence, tax is also split 50/50 between the investors and applied against your respective IRD numbers.
How this works in practice
If you are on different tax rates, the calculation of tax and payment at year-end will be calculated using the higher rate of the two investors.
This calculation is then passed to IRD, they will then split the calculation between the two investors and reassess the tax owing for each investor using their PIR. This ensures the correct tax is paid by each investor, but means the investor on the lower rate will have tax credits to set against tax obligations or receive a refund.
Can I change the split percentages?
You may choose to alter the split from 50/50 (for example change the split to 40/60), but this will need to be done through your tax return, through an IR3 form.