Can an iMac use a MacBook in sidecar or vice versa?

Hello, drm51.

No. Sidecar is designed to allow iPad to function as a second display for a Mac.

Please see this support article –

User your iPad as a second display for your Mac with Sidecar - Apple Support:

https://support.apple.com/en-us/HT210380

As mentioned SideCar is for iPad usage as displays only.

However, depending on your model of iMac and MacBook Pro, you may be able to use Target Display mode to have the iMac as a display for the Macbook Pro.

See here for details: link-> Use your iMac as a display with target display mode - Apple Support