When multiple threads allocate memory on the heap, they can access and modify the same memory locations, allowing them to share data between each other. However, this can lead to race conditions and other synchronization issues if the threads are not properly synchronized. To ensure proper synchronization, threads can use synchronization primitives like mutexes, semaphores, or atomic operations to coordinate access to shared memory. Additionally, threads can use thread-safe data structures like queues or lists to safely share data between each other without causing conflicts. Overall, it is important for threads to coordinate and communicate effectively to ensure safe and efficient memory sharing on the heap.