That's very odd. I tried two versions of what you said, loveSims. First I made a sim with two horses, two dogs and two cats and gave him two of each minor pet cage and had him stock them. This did not fulfill the LTW.
Then I gave a different sim (same LTW) two of each cage, had them stock them and then adopt two of each: horses, cats and dogs. This also did not fulfill the wish. I really thought this would, given how easy the Animal Rescuer LTW is.
The only way I was able to fulfill the wish was the old fashioned way: I did adopt six pets (seven, actually, as I was testing out my original question) and then spent nine days searching for the minor pets necessary to fill the terrariums.
As for my original question, my test says that yes, they must be owned simultaneously. I adopted two dogs and then gave one back before adopting my final horse. Having owned two dogs wasn't sufficient. Only when I adopted a third dog, bringing the current household number to two, did the wish complete. Unlike Metro's versions, I didn't teach any of my pets to hunt. I fed them in the morning and left them at home with a butler to look after them. My sim spent the rest of the day searching for minor pets. At first I collected anything and everything and sold it. By the end, I just collected gems and of course the minor pets.
I've also come to suspect there's a hierarchy of minor pets and their difficulty to find. Might finding them be a hidden skill? Birds are easiest, then turtles. Both of these appear right away when you start looking, though birds in larger numbers than turtles. Rodents were next. I had to capture several rodents before I saw my first snake. The same was true with snakes before I caught sight of my first lizard. Birds, turtles, rodents, snakes, lizards. Any remotely flat, large space is likely to contain a spawner, I found. Never did I find a minor pet on uneven terrain. I never found two of the same kind close together, but anywhere from 1/3 to 1/2 of my discoveries were followed by the discovery of another minor pet of a different species close by.
All in all, it took nine sim days, or about a morning of playing.
