I had a consulting call last week with a client who was thinking about starting a niche job board AND a career advice site that would help promote the job board.
They were going to launch both sites at once and wanted to know if I thought they should just put all the content on the job board and do it all on one domain.
They were a team of 3 people and had deep expertise and connections in the niche they were going after.
At the end of the call I felt comfortable in the fact they they could actually pull it off. They explained how good they were at content marketing and planned to use the career advice site, a podcast and a book to push people to the job board on an ongoing basis.
However if they were a one person startup with no experience in running a website I would have counseled them not to do it, stick to one site at a time.
Running 2 sites at once is a challenge. You have to split your marketing efforts between online properties and you can easily lose focus.
Start with one job board first, build the foundation over a year before you start thinking about launching another one. Two sites require a bigger team and more resources to make it all work in the long run.