Can I use a single macOS Mojave installer app on both iMac and MacBook I have both an iMac and a MacBook Pro. When I want to upgrade to macOS Mojave, do I have to download the installer in both machines? Or is it possible to copy it over to the other device and use the same installer?
Yes.  You can copy the installer to the other machine as it's "universal" to the Mac platform.

However, my suggestion would be to create a USB Installer instead.  This way you can download it, "move" it to the USB and then remove it from your installation.

You will need an 8GB USB flash, formatted with a GUID partition and HFS+.  Then execute the following command:

sudo /Applications/Install\ macOS\ Mojave\ Beta.app/Contents/Resources/createinstallmedia --volume /Volumes/USB\ Installer --nointeraction --downloadassets


Where /Volumes/USB\ Installer is the name of the USB flash drive you've inserted.