First off, Soresu was developed to combat blasters. Certainly this defensive techniques can also be used in lightsaber combat, but since when do those trained to use a lightsaber only ever use one form?
From the description section on Soresu:
Soresu focused almost entirely on self-defense, often at the expense of offensive capabilities.
The keywords would be "almost" and "often". It doesn't mean there's no offense involved.
Imagine if an MMA fighter (see Mixed Martial Arts) went into a fight with the intention to only lay on the ground and wait for their opponent to get on top of them for a ground fight? Especially in the case where the fighter laying down is a much better ground fighter than the other, whom may be a better fighter standing up. It's certainly happened in the ring, and it is a bit silly. So what happens? Well, in a sanctioned match, if the standing fighter doesn't want to go to the ground against his opponent, then the referee will have the fighter on the ground get up. The match needs to move forward and make progress.
In a lightsaber duel then, if the intention is to defeat your opponent, then something needs to happen. They're not both going to stand their ground and wait for an attack only to maybe counter.
So to answer your question directly, yes, Soresu can certainly be used against another that uses Soresu, but it's highly unlikely that it would be the only form used in combat.