I would have done the same as you to get the work done promptly and assuming that reimbursement would be forthcoming one way or another.
It seems to me that the two owners corporations should have each claimed against their own insurance (who might be the same insurer since there are not many doing strata insurance). Those insurers could then have recouped their costs from the driver or the driver’s insurance since the driver admitted liability.
However, ultimately, it should be the driver’s insurance paying for the whole thing. So, if the neighbours are uncooperative and your OC is out of pocket, then I would be first asking the driver’s insurer to pay up and, if that doesn’t work, going to the Tribunal for an order that the driver or their insurer reimburses your OC for the full cost of the fence.