If you use an immersion blender for a more uniform consistency, you do not need to skin the peaches. If you plan to leave the jam chunky, I recommend skinning them first. If you don't like seeds in your jam, you'll want to use the blender and then press the jam through a sieve to remove the raspberry seeds. Personally, I don't mind the seeds and I skip this step (which is kind of a pain).
I would recommend using pectin in this, as both raspberries and peaches are very juicy and not high in pectin themselves. Alternatively, you simmer it for a really long time and end up with less of the final product. If using pectin, follow the instructions on the package for how much to use per unit of fruit and when and how to add it.
8 cups sliced peaches (thawed if frozen)
4 cups raspberries (thawed if frozen)
1 1/2 cups sugar
1/4 cup lemon juice (optional)
Pectin per package directions
Combine the peaches and raspberries in a Dutch oven or other large pot. Bring to a boil, then simmer until cooked through. Use an immersion blender to puree the fruit. Stir in the sugar (combined with the pectin if that's what your pectin instructions call for; otherwise stir the pectin in afterward). Bring to a rolling boil, then remove from heat.
This recipe is suitable for canning, processing 1/2-pint jars in a boiling water bath for 5 minutes. Otherwise, refrigerate or freeze.
Makes 9-10 half-pints (less if you opt to simmer instead of using pectin to thicken).