Step 1Cut the cactus pears in half lengthwise. Using a spoon, scoop the flesh into a bowl. Crush the flesh with a fork. Fit a food mill with the medium disk and pass the flesh through the mill to separate the pulp from the seeds, scraping the underside of the mill to capture any purée clinging there. Discard the seeds. Or use a sturdy rubber spatula or spoon to push the pulp through a coarse-mesh sieve or colander to remove the seeds, scraping the underside to extract as much purée as possible. You should have 2 to 21/4 cups (480 to 540 ml) purée.
Step 2Mash the raspberries through a mesh sieve (or a food mill fitted with the fine disk) set over a small bowl, using the back of a spoon to push the pulp through and working to extract as much as possible. You should have about 3/4 cup (180 ml) purée.
Step 3Add the raspberry purée, sugar, lemon juice, water, and kirschwasser to the bowl holding the cactus pear purée and stir to mix well. Cover and chill for several hours or up to overnight.
Step 4Freeze in an ice cream maker according to the manufacturer's instructions. Pack into a chilled container, cover tightly and freeze, preferably for several hours, before serving. Remove from freezer about 10 minutes before serving to make scooping easier.