Certainly! Here is a test case generator for the given problem statement:

```python
import random

def tcgen():
    # Generate values for X and Y within the given constraints
    X = random.randint(1, 10**18)
    Y = random.randint(X, 10**18)  # Ensuring Y >= X

    return X, Y

# Example of how to use the generator
if __name__ == "__main__":
    for _ in range(5):  # Generate 5 test cases as an example
        X, Y = tcgen()
        print(f"Input: {X} {Y}")
```

### Explanation:

- **X**: This is the starting point of the sequence. It is chosen randomly between 1 and 10^18.
- **Y**: This is the upper limit for the sequence. It is chosen randomly between X and 10^18 to ensure that Y is always greater than or equal to X.

This generator will produce random valid inputs for the problem according to the given constraints.