I would also suggest you buy two PLASTIC muzzles. Metal muzzles are very prone to injuring others and you do not want a metal muzzled dog bumping his schnoz into you. Whenever they're within reach of each other, always muzzle them.

When you introduce them, you always go onto neutral territory, or a place that your resident dog has not claimed his territory. This can be the sidewalk, your neighbor's yard, the park, etc. Hold the leash with the proper grip- with your hand outstretched and your palm to the sky, place the leash flat against your palm. The part of the leash that is connected to the dog should be below your hand. Now, put your thumb against the side of your hand so that it lays on top of the leash. Take the free end of the leash and lay it over your thumb. Now, both ends of the leash are pointing towards the dog. Place the rest of your fingers over the leash. This is the proper grip that military dogs and police dogs use. I can take pictures of the proper grip if this doesn't make any sense. This grip is very important when dealing with two powerful dogs who tend to resort to scuffles such as yours.

When the two dogs meet, stand with your feet planted shoulder width apart. Move as the dogs move. If your daughter's dog moves left and they are now standing at 3 o' clock. Move to *your* left so that you stay synchronized. When your daughter is at 12 o' clock, you're at 6 o' clock. This way, the leashes will NEVER tangle, and you can quickly pull your dogs out. As Pit Chick mentioned, always be aware of the reactions they are giving/receiving. Look for signs of trying to heighten themselves (i.e. the standing forward and staying on their toes, ears up right, chest out, tail up) and signs of submission (i.e. lowered ears, head down, slinking). Praise continually for not growling or showing signs of aggression. After a few minutes, you can pull them out and lavish them with treats.

I highly do not suggest chaining your dogs outside. A chained dog in new territory is a fearful dog. My two dogs were never separated via one being in the house, one being out the house. They were both in the house 24/7 except for potty breaks and walks. Of course, yours are not reliable enough, yet, so you'll have to crate them or separate them in different rooms.

The alpha dog (doesn't necessarily have to be male) is the dog that is above the rest. S/he gets the toys and food first and s/he tends to be given more attention than the rest. We reinforce this to prevent scuffles because new dogs tend to want to challenge the pack hierarchy by scuffles. Usually, the resident dog is alpha, so if he is, I would continually give more attention to him and reinforce the idea that he is alpha over your daughter's dog. Above all, remember to make interactions between both dogs enjoyable! Praise and treat everytime they're in the same room and they're not fighting. Hopefully, all this tension will fade away.