Polite by default
The Sheltie was shaped on Shetland crofts as a small, economical sheepdog, and the temperament that came with the job — watchful, biddable, keen to keep everything in order — makes him one of the easiest dogs to add to an existing household. Shelties are reserved with strangers but usually courteous to the point of formality with other dogs: no barging, no mugging, no contests over status. Many pick a favourite housemate and shadow him devotedly. If your existing dog is gentle and your household is calm, the Sheltie will likely consider the arrangement ideal.
The referee instinct
Herding drive does not clock off indoors. A Sheltie watching two housemates wrestle often cannot help himself: he circles the game, darts in, nips at the heels of whichever dog is running, and provides a running commentary of sharp barks — behaviour the players may read as an invitation or an insult, depending on their temperaments. Chase games are the accelerant. This is manageable — a solid recall out of the refereeing, and sanctioned outlets like structured fetch and training games, give the instinct somewhere legal to go — but a home that wants a Sheltie should expect to supervise the pack's rowdy moments more than most.
Sensitivity to pack weather
The Sheltie's less visible trait matters more. Shelties absorb tension: a Sheltie living with two dogs who scrap regularly may never be touched by the fighting and still pay for it — going off his food, hiding during raised voices, dreading the garden at certain hours. Choosing his housemates is therefore a welfare decision for the Sheltie himself. One boisterous, wrestle-all-day companion who constantly triggers refereeing and alarm is a genuinely poor match, however well the two dogs appear to get on in short meetings.
Noise, coats and quiet corners
Expect the barking to be contagious in both directions — the Sheltie recruits and is recruited — and expect a double coat shedding into every shared bed and blanket in the house. What the Sheltie asks in return is modest: a quiet retreat the other dogs respect, predictable routines, and mental work of his own, because an unemployed Sheltie appoints himself to full-time pack management.
His training, protected on the rota
In a two-dog home the steady dog's needs are met almost by accident while the Sheltie's trick training and scent games are forever the thing that slips. KinsLe's Care Teams lets you put the Sheltie's ten minutes of brain work on the rota as his own assigned task, marked done with a timestamp, so his programme survives the weeks when the other dog needs more of everyone.