In addition to grouping horses for Trackwork (see How to group horses for Trackwork) you can also pair or link two or more horses together if they are performing the same Work.
You can link horses for Trackwork via the Schedule > Work screen.
Select the horses you wish to pair up or link, and click the Link button.

Horses may have different Pre-Work and Post-Work scheduled, but if they are performing the same Work, they can be linked.
Once linked the Horses will appear next to each other with a link icon between them.
The Unlink button allows you to quickly unlink ALL horses currently linked.
Where three or more horses are currently linked to work together, the Unlink action icon in the right hand side of the screen unlinks just the horse selected - not all the horses.