Where did you send them?
I personally don't like sending them to Sanctuary or another large settlement because I have a hard time finding them again.
I've lost Dogmeat once and had him turn up back at the Red Rocket gas station.
Whenever I try to get a new follower it's always asked me where to send the previous one. Which is why i'm not sure how you could have two companions right now.
I don't send Dogmeat to large settlements, e.g. Sanctuary Hills.
I make sure there is a Doghouse at all places I send him.
Similarly, I'll send companions to small settlements close to the City so I can find them.
It would be nice to have the option to send companions to other 'suitable locations'
but I'm sure there will be a mod for that.
Dog meat, if sent to sanctuary, SHOULD be in a small "dog house" north in Sanctuary.