Can I use a single macOS Mojave installer app on both iMac and MacBook
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.