BannerCells lets you have a group of cells sharing the same backgound color scheme (gradients or whatever) or bitmaps. You can see samples in Syncfusion\Essential Suite\Grid\Samples\Quick Start\BannerCells.
You can use a covered cell to treat a range of cells as 1 cell which would share a single border like a single cell. But if you actually want to access the cells in the range, then there is no built-in support for this.
Here is a sample that treats the first 6 rows as 2-row sets, outlining the two rows with one border. But it does this from code.