Lavender (Lavandula) and heather (Calluna) are two of the easiest plants to grow, depending on your location and climate zone. They both spread out considerably and, if left alone, can end up growing together. When you want to untangle the plants and replant in new spaces, wait until fall or very early spring to dig them up.
Water the area around the plants you want to separate. Let it soak in and wait for about 30 minutes afterward to begin digging.
Dig up the plants. You can do this with your spade or use a pitchfork if you want to eliminate too much extra dirt coming up with the roots.
Turn the plants upside down. Pull the lavender carefully through the heather branches. Do this until you have removed all plant parts, including runners. The runners are the lower-branching offshoots that are putting down roots. These are great for starting new plants.
Replant the heather where it was growing if it was performing well. Water it in well. Cut the runners from the lavender, especially if the plant is old and too woody, and lay the runners into new holes in a separate area -- at least 6 feet from the heather. Ideally, set up a completely new growing area for the lavender plants.
Water the lavender in well. Water both the lavender and heather to keep the ground moist for a few days afterward, to ensure the plants are getting what they need. Then just focus on the lavender -- especially if the heather is older and more established.
Wearing gloves when dealing with woody or poky plants can help, as can long sleeves. If you have too much mud when you begin digging, wait and let the ground dry out a bit more.
Don't feed nitrogen to heather plants or you'll likely lose them.