I have a setup of a MacBook Air (2013), an iMac (2009), and a regular external monitor. I’m able to connect either the iMac or the monitor to the MacBook and use them as an additional screen. Is there way though to connect both of them at once, so that I have two external monitors managed by the MacBook? Obviously, the MacBook only has one Thunderbolt port, but there’s an extra one available on the iMac. Any way to utilise it in this case?
Yes, but you need a USB display adapter (any DisplayLink adapter, like this one should do) for your MacBook Air, the iMac can’t do passthrough.
The 2009 iMacs don’t have Thunderbolt, only mini-Displayport. Even the later iMacs with Thunderbolt can’t do passthrough with their second Thunderbolt ports. Therefore you need a second display connection on the source Mac. Since the MBA only has one, you’ll need to use a USB-based adapter. There are many options, pretty much all of them using the DisplayLink technology. It works well for 2D stuff, but don’t expect to do any gaming or other 3D work on it.