dearcharlie wrote:
I have a Thunderbolt 2 to USB-C lead and that doesn't seem to be able to connect them.
Are you talking about an Apple Thunderbolt 3-to-2 adapter? That might let you establish a data connection (for IP-over-Thunderbolt, or possibly for Target Disk Mode), but it wouldn't let you use the MacBook Air as a second "hardware" monitor.
Or are you talking about a USB-C to Mini DisplayPort adapter? The Thunderbolt 2 port on the 2017 MacBook Air can double as a Mini DisplayPort output, but it is not designed to accept Mini DisplayPort input.
Either way, the MacBook Air is not designed to act as a "hardware" display, so the best that you could hope for is some software solution. Given how cheap 24" 1920x1080 monitors with IPS panels and near-100% coverage of sRGB are these days, it's probably not worth spending too much on something like Duet Display or Luna Display just to reuse an old 13" 1440x900 laptop screen. (And a real monitor with multiple inputs could potentially serve as a second screen for both of your Macs on an as-needed basis.)