I don't recommend partitioning any drives, but especially a boot drive since users always select the wrong sizes for at least one of the partitions as users tend to run out of storage space on at least one of the partitions. When users go to resize partitions they end up losing access to their data and many times prevent either macOS from booting. When this happens you usually need to erase the entire drive to start over again.
Plus macOS Lion will not be able to recognize the newer APFS volume used by High Sierra so you will not be able to use the Lion Startup Disk System Preferences to select High Sierra as the boot volume.
I would suggest you instead use an external USB3 drive for your macOS Lion installation.
If you insist on dual booting on the internal drive, then you will need to use Disk Utility to create a new partition. Within Disk Utility you may need to first click on "View" and select "Show All Devices" so that the physical drive appears on the left pane of Disk Utility. Select the physical drive and click on the "Partition" icon followed by clicking on the "+" icon to create a new partition and resize it as desired before clicking "Apply".
Make sure to have a good backup before attempting to create a new partition. Plus make sure to have frequent and regular backups of both partitions and any external media which contains important and unique data.
And be prepared to erase everything and start over at some point when you realize you don't have enough free storage space on one of the volumes or when you realize you no longer wan to use macOS Lion.
