A heartbeat not being clearly seen on one twin at this stage can sometimes come down to positioning or image angle rather than the pregnancy itself, but it can also mean that twin isn't progressing, and only your follow-up scan can tell which.
Next Steps
Your six day follow-up scan is exactly the right next step, and it's worth having it done at the same clinic if possible for consistency.
Health Tips
Please try to hold off on drawing conclusions either way until that scan, as hard as that is to do.