Adobe Systems has been putting significant effort into allowing its Creative Suite of products to be used with only one monitor. The effort has been touted in new feature press and interestingly their workspace optimization has been focussed on single monitor use. There are some examples cited below.
Anyone who has been accidentally tripped up CS UI's elaborate minimization buttons can see that its been a priority. I would question it - as dual or even trippletriple monitors has proven to be the norm amongst professionals.
Dual monitors allow you to have an unimpaired view of the USer'sUser's Experience. The only downside I see is that if the monitors are too large - there is a natural momentum to push the size envelope. Many UI professionals have one small monitor - 17 inches to keep them in what is increasingly a smaller space in terms of real estate, considering the growth of mobile and tablet devices.
Illustrator workflow: http://help.adobe.com/en_US/illustrator/cs/using/WS714a382cdf7d304e7e07d0100196cbc5f-633aa.html
Premier workflow:http://helpx.adobe.com/premiere-pro/using/workspaces.html
Photoshop work flow: http://help.adobe.com/en_US/photoshop/cs/using/WSfd1234e1c4b69f30ea53e41001031ab64-750ca.html
Illustrator workflow: http://help.adobe.com/en_US/illustrator/cs/using/WS714a382cdf7d304e7e07d0100196cbc5f-633aa.html
Premier workflow: http://helpx.adobe.com/premiere-pro/using/workspaces.html
Photoshop work flow: http://help.adobe.com/en_US/photoshop/cs/using/WSfd1234e1c4b69f30ea53e41001031ab64-750ca.html