In a small saucepan over medium heat, cook strawberries, sugar and water until mixture is bubbly and strawberries are very soft.
Transfer mixture to a blender or food processor, along with bananas.
Blend until smooth.
Preheat oven to 170 degrees. Line two baking sheets with silicone baking mats or parchment paper.
Divide mixture between pans. Use an angled spatula to smooth out as best you can. It doesn't have to be perfect but the more even the mixture, the better.
Place in oven for roughly 3 hours. Time will vary depending on the thickness. After each hour, rotate pans 180 degrees and switch racks.
Leather should be slightly tacky when done, but not sticky or wet at all.
Transfer to a cutting board, to cut into desired strips. Wrap in wax or parchment paper and store in an airtight container.