70 and 2/3 (Eruvin 57a)

The chachamim in the mishna on 57a say that the only time you get 70 and 2/3 amos extension on a city is when there are two cities. Then Rav Huna in the gemara says that between two cities you get 141 and 1/3 amos because each city gets the 70.7 amos. Does this mean that there is never a time that you'll only get the 70.7 amos?

Tosafos in d"h Rav Huna, explains that when the chachamim say you only get the karfef between two cities they don't mean between two cities but between two things. In other words a house gets the karfef also. You don't need a city at all. If I have a house and my neighbor has a house 70 amos away then that's OK and we can start our techums only after the other's house. However, when Rav Huna says that both get 70.7, he means that only when there are two cities. So between a house and a city you only get 70.7 just like between two houses but between two cities you get 141.3 (and I think that according to Tosafos that is only true if they're both walled).

The Ritva brings up another interesting point. Let's say you have a city with a house 50 amos away and then another city 140 amos from that house, can you say that it's all one big city? Or do you have have less than 140 amos between the cities and the house doesn't count to extend the city for that purpose? The Ritva says the house doesn't count. The Maharam M'Rutenberg disagrees.