Use a Macbook Air as an 'extended' screen for an IMac

Hi There,

I have an iMac, with a Thunderbolt out, and wanted to connect my Macbook Air, also with a Thunderbolt out as a 2nd monitor, so that I can work with Photoshop on the iMac and have my tools as an extended workspace.

The specs are :

iMac : iMac (21.5-inch, Mid 2014)

Video Support :

• Simultaneously supports full native resolution on the built‑in display and up to a 30-inch display (2560 by 1600 pixels) on an external display
• Support for extended desktop and video mirroring modes
• Support for Target Display Mode via the Thunderbolt port using a Thunderbolt to Thunderbolt cable (sold separately)

Macbook Air : (13-inch, Early 2014)

Video Support :

• Dual display and video mirroring : Simultaneously supports full native resolution and up to 2560 by 1600 on an external display
• Thunderbolt Digital Video Output

I just updated both to be on the Catalina OS.

They both show that it's possible from a hardware standpoint, but when I go into the settings in 'displays', and try 'option', 'detect displays' they don't see each other. I have no ability to add 'extended desktop' and access the 2nd monitor.

Is it my thunderbolt cable, or are they not compatible? I purchased a regular Thunderbolt, not a Thunderbolt 3 cable.

Thanks for any insights and help, it's driving me a little crazy.

Kat