Depends on how you look at it really.
If you are looking at resources utilization, you should be opening your second hub very close to your first location-wise, at the time when all airports within the range of your first choice of regional aircraft are served. Even though you might have to save a little bit to afford opening the hub, doing this would boost your margin instantly because you will be using the gate slots that are formerly empty, with the hub gates as the only additional costs.
If you are looking at it from a cash-flow perspective, then what Matias said.
If you are looking at it from a ROI perspective, you would need to set up an excel and plug in data based on experience / projection to decide the exact point between the above two (as in when the wait to save up, and hence lacking a instant return on the cash-on-hand, would provide a large enough additional ROI to put you in a better place than if you re-invest in new planes at hub 1 instantly).
If you are looking at it from a competition perspective, you might want to stake your claim on some airports at the earliest possible, or you might want to have a hefty reserve before swooping into certain airports and launch a large network in a very short time.
Not helping... I know.