Parallel Transmission Lines
Placing two or more transmission lines (transformers, cables, ...) in parallel is a common way to obtain increased capacity and redundancy against single failures. It also lets you take one line out of service for maintenance, while the second line carries the load.

Mathematically, you would obtain the same results by combining the parallel transmission lines, but you would lose the ability to simulate a scenario where only one of the two transmission lines is in use. Simulating contingency scenarios ("what happens if one line has a fault?") is one of the main reasons we do power systems analysis, so I would tend to model the two transmission lines as two separate elements.