The Ruthenians were a group of East Slavic people in Kievan Rus that are not what we would call "Russians" today.
Generally the Ruthenians is seen as the people of all the Kievan Rus, and indeed the original source of Russian, Russians and Russia.
The term refers mainly to Belarussians and Ukrainians
And Russians, at least when you speak in historical terms.
the two largest groups who had a common "Ruthenian" language.
This is correct. The Old East Slavic language spoken in Kievan Rus is seen as the source of Belorussian, Ukrainian and Russian, and the split between Russian and what is now called Ruthenian came first, and Ruthenian later split into Belorussian and Ukrainian.
The two peoples were together as part of the same country under Kievan Rus, the Duchy of Lithuania, the Polish-Lithuaninan Commonwealth, Tsarist Russia, and the Soviet Union.
Well, no. There was one people: The Rus, also later known as the Ruthenians, living roughly in the areas and state we now call Kievan Rus. But that state collapse in around the 12th century, and there was much fighting, and Mongols and stuff going on. It ended up split between Russia, Lithuania and Poland.
1569 the Polish-Lithuanian Commonwealth was established to join two separate countries in a Union. But they were still at the least two separate administrative areas, the border going not entirely unlike the modern Ukraine/Belarus border.
When the areas fell under Tsarist Russian rule, this separation continued, but the areas were now called "White Russia" (Belarus) and "Little Russia" (which in the 19th century came to instead be called Ukraine).
After the Russian revolution, both these areas wanted independence from Russia. Exactly why, and why they didn't join together against Russia, can be debated. But it can't be expected that they would join because of some shared heritage from the Ruthenians, because they share that heritage with the Russians that they wanted independence from.
In any case these people did not suddenly pop up into existence in 1917. The split of the Kievan Rus into three (Russia, Ukraine and Belarus) goes back to at least the early 16th century, albeit with differing names and constantly shifting borders.