While it may be possible to plant blackberries and raspberries together, it would be quite the challenge for even the most seasoned planter with a green thumb. Because these two plants have such different growth habits, it is unlikely that they would survive being planted together.
Those who want to try it though will need to go through a few steps to ensure that the process is successful, such as testing the pH of the soil where these plants are going to go.
You will need to keep in mind that both of these berry plants do better in soil that is slightly acidic. The next thing you will need to do is to add ground rock sulfur and mix it throughout the planting bed if you discover that the overall pH of the soil is higher than 6.0.
After you do that you will need to create numerous mound row beds which are around 10 inches high and anywhere from two to three feet apart; you will do this by tilling the soil as well as adding compost as needed in order to increase the overall nutrient value and drainage.