I presume we're talking about a cut string stair (because if it isn't a cut string, then the ballusters just get spaced out along the top of the string, with no relationship between them and the treads). They seem too close because the ballusters are so big. I guess they are turned, but with square ends? Never mind all that, if it is a cut string stair then the answer is yes, to comply with regs, they need to be two-to-a-tread, simply because if they were one to a step the gaps would be about 180mm (they need to be less than 100mm).
BTW, be very careful with this 9" tread" thing. If the distance from the front of the nosing to the back of the step is 9", then your stair isn't in compliance with the regs. The minimum going is 220mm, and your minimum overhang of the nosing is 16mm. That means the narrowest your treads can be is 236mm, and if you really do have 9" (228mm) then your goings are too narrow. If the 9" you quote is nosing to nosing, then you're fine.
Funny thing is I am sitting here drawing my own cut string stair as we speak.
