should add psmouse.synaptics_intertouch=1 to GRUB_CMDLINE_LINUX in /etc/default/grub , run update-grub as root or sudo, and reboot. ideally you wouldn't make this permanent at first, but type it in via grub menu “linux” line
you can edit that grub entry, and find the “linux vmlinuz…” line, and at the end of it add that psmouse thing then hit F10 to continue booting that entry this is temporary altering the kernel “cmdline” so if something breaks, you can just reboot. alternatively, if you add it to /etc/default/grub , it's permanent so if something breaks, you have to edit grub menu to remove the entry. other way around.