I think it really depends on how the townhouses are designed.

There's one I know near Putrajaya where a cousin of my friend stays in which is 2-storeys designed like a Semi-D. Ground floor is one unit and the 1st floor is another unit. Each have 2 carparks, so there's a partition wall in the middle to separate the units' carparks on the ground floor. Ground floor unit carparks are on the left partition and the 1st floor carparks are on the right partition. That friend stays on the 1st floor, which means her side have a staircase connecting her carpark to the unit's balcony.

The other one I know is Sommerset Close which is at Taman Pertama (or is that part considered Bandar Sri Permaisuri?) next door to Lido Residency. Some units come with only 1 carpark and there are others which comes with 2. From a few pictures I've seen of the subsale units, it looks just like how you described it where the 2 units share the same staircase.