Sure, let's create a test case generator for the "Land Distribution" problem. 

To generate the test cases, we need to:
1. Randomly choose values for \(H\) (height) and \(W\) (width) within the appropriate range.
2. Generate an orchard map with random characters ('@', '#', '*') and whitespace.
3. Ensure that the input ends with '0 0'.

Here is a Python function to generate such test cases:

```python
import random

def generate_orchard_map(H, W):
    plants = ['@', '#', '*', ' ']
    orchard = []
    for _ in range(H):
        row = ''.join(random.choice(plants) for _ in range(W))
        orchard.append(row)
    return orchard

def tcgen():
    datasets = []
    max_datasets = 20
    for _ in range(random.randint(1, max_datasets)):
        H = random.randint(1, 20)
        W = random.randint(1, 20)
        orchard_map = generate_orchard_map(H, W)
        datasets.append((H, W, orchard_map))
    
    # Ensure we end with the '0 0' line
    datasets.append((0, 0, []))
    
    return datasets

# Format the output as required by the problem statement
def format_test_case(datasets):
    result = []
    for H, W, orchard_map in datasets:
        if H == 0 and W == 0:
            result.append('0 0')
        else:
            result.append(f"{H} {W}")
            result.extend(orchard_map)
    return '\n'.join(result)

# Example usage
datasets = tcgen()
formatted_test_case = format_test_case(datasets)
print(formatted_test_case)
```

### Explanation:

1. **generate_orchard_map(H, W)**:
   - This function generates an orchard map of size \( H \times W \).
   - Each cell in the map is randomly chosen from the set of valid characters: '@', '#', '*', ' '.

2. **tcgen()**:
   - This function generates multiple datasets (up to 20) with random values for \( H \) and \( W \).
   - For each dataset, it creates an orchard map using the `generate_orchard_map` function.
   - It ensures that the input ends with the line '0 0' to indicate the termination of input.

3. **format_test_case(datasets)**:
   - This function formats the generated datasets into a string that matches the expected input format for the problem.
   - Each dataset is represented by its height and width on a line, followed by the orchard map rows.
   - The output is joined into a single string with newline characters.

You can run the `tcgen()` function to generate random test cases and then format them using `format_test_case(datasets)` to get the desired input format for testing.