RoamEO :

The new RoamEO has a few tricks up its sleeve.
RoamEO GPS Pet Collar Tracks Two Dogs at Once


For starters, it can track 2 dogs per system. Using its LCD display, the RoamEO lets you see how far a distance is between the RoamEO Pup which you are using, and the RoamEO Collar worn by your pet. You can even see the direction they are currently traveling in.
The RoamEO’s total range capability is between 1 and 3 miles away and it can even work indoors.