Introducing two unfamiliar adult dogs can be
tricky.
The key to good intros is to do it in stages.
Start with the dogs meeting on neutral territory--a park or another
person's yard--somewhere where there are few distractions, not that
many other dogs and people, and a place about which neither dog
will feel territorial.
Keep both dogs on-leash during the entire time. Have one person
hold each leash and let the dogs walk around each other. IMPORTANT:
keep the leashes slack and keep your own apprehension controlled.
Tightening the leash can increase aggression as the dogs can feel
the increased tension in the handler's body.
As the dogs walk around each other, watch body language. Don't
allow any kind of aggressive stances: staring, body positioned more
forward than normal, etc. If these happen, go into a "walk
away" with the dog and then try again. Also watch for good
body language: play bows, playful antics, etc.
Let the dogs smell each other's pee/poop areas, then let each dog
smell the other's hindquarters, making sure not to let anyone get
tackled unexpectedly, obviously.
As things get comfortable, move everyone to a non-neutral outdoor
place: the new owner's backyard, for example. Do the same thing
as at the park. If things look good, drop the leashes, but keep
them on the dogs, and let the dogs wander the yard.
Keeping the leashes on helps to maintain order. If the dogs fight,
two people can pick up a leash and walk in opposite directions without
getting their arms and hands in the fight and risking an unintentional
dog bite.
Once the dogs look okay outside, do the same kind of thing inside:
leashes on, dogs inside. If they look okay while the handlers are
holding the leashes, drop the leashes, but keep the dogs supervised.
Also, BEFOREHAND, pick up ALL toys, rawhides, food bowls, etc as
you don't want either dog to suddenly feel threatened by the other's
presence. Don't let either dog get near the other's crate, if you
use crates.
Finally, even if things look great, you may want to keep the dogs
separated while you are absent from your home, until you feel they
are fully comfortable with each other.
Remember your Shibas might get along right away, but the introduction
period might also take a few days, weeks, or even months for everyone
to feel comfortable. Be patient!