Hi! yes, I've done 2 puppies at once. Actually, I had 3 dogs 1.5 yrs and younger. Gunther was 1.5 yrs old when we got Nigel and Posey at 6 mo old (although they had stayed with us a month when they were 12 wks old)
I think that if your mini aussie is as well trained as you say, you can definitely do another puppy and it wouldn't be much of a problem. Gunther was pretty much trained by then, but Nigel and Posey had almost no training other than being potty trained (which they completely forgot when we moved a few days later). I don't think i will ever do 2 dogs the same age again, at the same time, with both of them being puppies. Of course I had other things going against me as well..........
..our housing situation was up in the air so i had to stay with my folks for a year and a half while hubby looked for a house in another state.
As for dachshunds....
........is it a long haired, short haired or wirehaired? Being a mini, even if its a short haired, it's attitude should be better than the standard siz. the long hairs are the sweetest, with the wirehairs being more terrier like.
and even though they are technically hounds, they aren't hounds in the general sense of the word, b/c they are earth dogs. So many like to dig. Since you don't have a yard that shouldn't be a problem.......
..lol. They can be very protective of their humans so socialization is key unless you want an ankle biter. Training can be a challenge, as they aren't as eager to please as the aussies are.
The major thing you want to think about though before getting one of these breeds is the health issues that go along with them. Mainly, serious back problems. They are very commen. Dachsies cannot jump up, or down, from furniture. They must be trained to use ramps or steps. No running up and down steps. No jumping up or down period. If I had to take a guess, I'd say that serious back problems, the kind that require surgery, occurs in about 60% of all dachsies. i don't know of any that have never had any back problems at all. This is the one reason although I like the breed, it is too much of a liability for me to own one, and too risky.
Also, depending on the color, there can be other problems as well. Double dapples and piebalds i believe can get inherant skin problems.
but they are a spunky and fun and will definitely keep your life interesting.