Why not make this accept a variable number of arguments?
```python
def load_requirements(*requirements_file_paths):
    requirements = set()
    for file_path in requirements_file_paths:
        requirements.update(
            line.strip() for line in open(file_path).readlines()
            if is_requirement(line)
        )
    return requirements
```

That way, you can call `load_requirements("pre-requirements.txt", "requirements.txt")` and `load_requirements("dev-requirements.txt")`, and it should do what you want. Note that in this specific example, I've changed the return types to returning a set instead of a list -- you can easily convert that set back to a list just by calling `list()` on it.