You shouldn't assume that alliance commitments (or, for that matter, obligations under international laws) are automatically honored, even when they are against the best interests of the country that should honor them.
There is a middle ground between, let's say, "complying with your alliances only when it suits you" and "mindlessly complying with your alliances even when that means national suicide", and most countries work within that middle ground.
In that time frame, there were lots of situations where both logic and a strict application of the alliance systems should have yielded outcomes that, on the contrary, did not come to pass. The French should have supported the Czechoslovakians back in 1938. But they managed to wiggle out of their obligations.
The British commitment provided that it would kick in only if Poland had "become engaged in hostilities with a European Power in consequence of aggression by the latter", and Art. 2 mentions a threat so grave that Poland would have "considered it vital to resist it with its armed forces." (The French treaty had similar clauses).
As it happened, the Polish government's stance prevented the activation of the commitment (even though individual clashes did take place, they were under the initiative and responsibility of local Polish commanders, so Poland as a whole not having declared war nor ordered those fights, was not "engaged in hostilities" with the SU, nor resisting with its armed forces). So the British (and French) could breathe, not having to declare war on the SU.
If the Polish government, on the contrary, does declare war and order its remaining troops to fight, the British (and the French) might still wiggle away, if they decided that was necessary. For instance, they might claim the Polish declaration of war was only formal, without actual "hostilities" (that would be contrary to the truth, but then, who would disclaim that? The Soviets?); or they might state they accepted the Soviet version of the events, and that there was no "aggression". It would be hardly honorable, but as I said in the beginning, you shouldn't rule that automatically out.
The same applies to the Soviet position if we assume Britain and France do declare war. Historically, we know that they did against Germany – and that did not bring about an all-out offensive, while the French had a border in common with the Germans. So I fail to see the Soviets (sitting back out there, where the Allies would need strong sea power projection capabilities to annoy them) as feeling they absolutely immediately urgently needed an ally to fight back a Phony War.
Of course there were plans to strike back at the SU, some will say; in Scandinavia, at Baku. Yes. And there is a reason if they never came to fruition. In this scenario they are a bit more likely, since Britain and France have declared war on the SU. This still does not mean they will bring about an all-out war. These don't necessarily happen between half-hearted opponents, the contrary is more likely. Both the Soviets on the one side, and the British and French on the other knew perfectly well who was the loose cannon of Europe and did not need to give it more friends.
For instance, what would have been more logical than a state of war between Japan and the SU in 1942? The system of alliances dictated that. Yet you know how much more time it took. Even in presence of a DoW, what happened on the German-French border in the winter of 1939 is not the only example of Phony War. Britain bombed Finnish territory at Petsamo in 1941, without a DoW. The Finns should have declared war, but did not, they knew very well it wasn't in their interest. The Brits took another five months before finally declaring, and did nothing concrete. You might study the interesting story of Bulgaria's diplomatic relations in WWII, it's a lesson about the difference between a formal state of war and actual war being waged.
In short:
- it's unlikely that the Poles declare war against the SU, it wasn't in their interest;
- if they do, it's unlikely the British and French follow suit, for the same reason;
- if they do, it might well be a Phony war, followed by a "it was all a mistake" when Germany attacks in the East, instead of a real war;
- if it is, it's not a given the USSR a) decides to declare back, b) decides to wage actual war back against them, c) feels it needs an alliance with Germany.
Of course I'm not discussing the possibility of Germany _not_ attacking the SU at some point. You need to change German leadership for that, but if you do, the whole sequence of events in 1935-1939 gets changed beyond recognition, with the war itself becoming unlikely.