those are 2 REALLY different breeds. Is the Aussie not a good fit for your family? Having multiple dogs works best when they can play together and have similar play styles. Otherwise, you're doing twice the work. Greyhounds are almost NEVER off leash. - so if you're used to taking your Aussie on long hikes, it could be weird to have this on leash greyhound that cant go as far.
I like having 2 dogs but its more than double the work in many ways. They learn bad habits from each other at an astonishing rate. And at least in our case, they tend to work together to cause mischief - for instance, one pushes the chair out so the other can jump onto the table to steal food. You need more space in the car if you're traveling - if you're used to taking the dogs with you places, people may not be as willing to have 2. Boarding costs arent quite doubled but it is more expensive.
otoh, dogs are social beings - many enjoy having a canine playmate. Its also nice having a dog for each lap that wants one. and a dog for each child to walk.
dd1 10/05
dd2 11/09
and ... a mini poodle!