can you use another iMac as a second display?

Depends on how old the old iMac is.

The new iMac can use Airplay to Mac to use another Mac as a screen.

The old Mac needs to be a 2019 or newer model running Monterey.

If the old iMac is older than that, then no, it cannot be used as a screen for the new iMac.

in times past it was possible, but it is no longer possible with the newer Macs. A good display is not all that expensive.