In most bands, the tenor drummers are known as the "mid-section". When you add a bass drummer, the mid-section becomes the "bass section". Because both tenor and bass drums are pitched to notes in the bagpipe scale, they are able to provide both harmonic support to the pipes and rhythmic support to the snare drummers. The bass drum is tuned to "A" (I use quotes as bagpipe pitches are much sharper than most other instruments) and tenor drums are tuned to other notes in the pipe scale. The choice of tenor pitches varies from band to band and depends on the number of drummers available and the musical opinions of a band's musical leadership.
If you are interested in learning more about the tenor section, please click on the links below to a four part series discussing the mid-section in-depth: