The short answer to what you're asking is yes, the Soviet Union participated in the invasion of Poland in 1939, but before I fully answer your question, I want to point out a couple of errors here. First, the Weimar Republic didn't invade Poland; Nazi Germany did. The Weimar Republic ended in 1933 with the Nazi seizure of power (Machtergreifung). Second, framing it as being "turncoats" is ahistorical, because at the time the Soviet Union wasn't aligned with what would become the Allied Powers (i.e., Britain and France, who declared war on Germany after the invasion of Poland pursuant to prewar alliances). The cooperation between Germany and the Soviet Union was certainly shocking to the West given their previously tense relationships and their mutually exclusive ideologies, but there was no formal agreement between the Soviet Union and Western powers at the time, so it wasn't really a "betrayal" as such. In fact, the Soviet Union had been talking out of both sides of their mouth and actively negotiating with both the Western powers and Nazi Germany prior in 1939, before ultimately signing a non-aggression pact with Germany.

The Soviet invasion of Poland was part of a secret clause within the Nazi-Soviet non-aggression pact (sometimes referred to as the Molotov-Ribbentrop Pact after the two countries' foreign ministers), which had been concluded on 23 August 1939, a week before the German invasion began. The pact included the division of Eastern Europe into German and Soviet spheres of influence, with a tacit understanding that these spheres of influence would be gained through either diplomatic pressure or direct military action. The Soviet sphere of influence included the Baltic States (which had become independent after WWI), Finland, and Bessarabia (present day Moldova) and Bukovina (the present day border region of Ukraine and Romania), both of which had united with Romania after WWI. This division also included the partition of Poland into German and Soviet zones, which would establish the border between the two after the joint invasion of Poland (although it was subsequently adjusted a bit after the invasion, with the German border moving east in exchange for Lithuania becoming part of the Soviet sphere of influence). The German invasion started on 1 September (the conventional start date for WWII), while the Soviets invaded on 17 September.

Although the German and Soviet invasions of had been timed to coincide, and some coordination was suspected by Western intelligence, the secret provisions of the Molotov-Ribbentrop Pact weren't known outside of Germany and the Soviet Union until after the end of WWII. However, the division of Poland was announced publicly after the Germans and Soviets concluded the treaty that laid out their new borders on 28 September 1939.

Later that year, on 30 November, the Soviet Union launched an invasion of Finland which was known as the Winter War, which led to the cession of territory that was considered within the Soviet sphere of influence. On 15-16 June 1940, while the German invasion of Western Europe was ongoing, the Soviet Union occupied the Baltic States and forced the creation of Soviet-aligned puppet governments through fraudulent elections. On 28 June 1940, they sent an ultimatum to Romania that impelled them to turn over Bessarabia and northern Bukovina or face invasion, which Romania had no choice but to acquiesce to. These annexations were influential in pushing Romania to align with the Axis and eventually participate in the war against the Soviet Union, and the repressive Soviet governments in the Baltic States motivated nationalist groups to collaborate extensively with the Germans during the invasion of the Soviet Union; they would eventually be heavily involved with perpetrating the Holocaust in the Baltic region as well.

Of course, even as they were concluding a treaty, both Germany and the Soviet Union believed that war between them was inevitable. However, both of them believed the period of peace would be beneficial to them. They concluded a series of economic agreements between 1939 and 1941 under which the Soviet Union sent food and raw materials to Germany in exchange for military equipment. The raw materials were important for Germany's remilitarization, while the military equipment was important for the modernization of the Red Army. The Soviets had no illusions about Hitler's ultimate intent, but they certainly believed that war could be delayed long enough to allow them to be ready to defend themselves. The Soviets continued to try to mollify Germany up until the invasion in June 1941, but Germany had been planning and preparing the invasion since the summer of 1940. Even as the Germans invited the Soviet Union to join the Axis in the fall of 1940, they were preparing for what eventually became Operation Barbarossa.

Sources:

Bernd Wegner, ed., From Peace to War: Germany, Soviet Russia, and the World, 1939-1941 (Berghahn, 1997).

Text of the Molotov-Ribbentrop Pact (including the secret protocol) can be found here.