The Bi-Fold door configuration is the standard, and most popular, Fireplace Glass Door style. Each door is split into two glass panels which allow the doors to fold back on themselves to save space and avoid possible contact with any obstructions surrounding the fireplace opening.
Bi-Fold Doors are available with tracks to keep the doors in place. However, the "trackless" option is more popular as it allow you to open the doors completey to display the full viewing area of your firebox opening.
Twin Doors have a single glass panel on each door. There is only one seem in the center of the two doors which creates a clean look as there is less visible hardware in the viewing area of the firelace when the doors are closed.
When opened, twin doors will require more space as they do not fold. If there are any obstructions such as windows, sliding doors or furniture in the surrounding area of the firebox opening, you should consider bi-fold doors for safety and to reduce the risk of damage.