Does 越来越多的中国人步入二次购车的行列 talk about second cars or second-hand cars?
In fact, ‘二次购车’ might not refer to '2 car families', it would be 'previously bought a car and are replacing it' as you said in most cases, but I'm sure that it is not 'second hand cars', since 'second hand cars' is translated as '二手车'.

In my opinion, this sentence may translate as 'More and more Chinese people are buying cars for the second time , the market is becoming more and more subdivided.'