Maybe the time and effort demands of both wouldn't have allowed it but given how Patrick's personal debt grew, it seems it would have helped to do at least a little bit of consulting while he was working on Appointment Reminder.

Doing freelance / consulting work on the side while bootstrapping your product to profitability seems increasingly common now. Though this is definitely more sustainable in locales with lower costs of living while you also don't have to provide for a whole family.