In short, relations between Poland and Lithuania in the 1918-1939 period were not good, going so far as to say bad.
The two newly independent states really had border disputes that were largely unresolved, with the biggest concern being the status of Vilnius (or Wilno, as is known in Polish). Now the capital of Lithuania, it was occupied by Poland during the entire period, though the Lithuanians consistently advocated for its “return,” regarding their de facto capital, Kaunas, as a temporary solution.
It is worth noting that ethnically speaking Vilnius was not an ethnically Lithuanian city; I will have to confirm the numbers, but it was mainly Jewish and Polish, with Lithuanians a very distant third, or possibly fourth (Belarusians and/or Russians may have been up there). This was an argument the Poles used to keep control of the city, in that it was hardly Lithuanian so there was no reason under the logic of the era (national self-determination) to give it to Lithuania. That Jozef Pilsudski, the de facto leader of Poland during much of this era, was born near Vilnius is arguably also a factor in the dispute. Regardless, the aftermath of the Polish-Soviet War of 1919-1920 confirmed that the city would remain within Poland.
The result was that neither side established diplomatic relations during this era, with neither side willing to concede on the Vilnius question. The idea of unifying the two states was thus inconceivable, and would not really be resolved until the post-Soviet era, when the disputes were finally settled.
I have some reading material I can add, I just don’t have it handy right now. Once I get home I’ll edit this post with those.
Edit:
As promised, some sources to consider:
Jerzy Borzęcki, The Soviet-Polish Peace of 1921 and the Creation of Interwar Europe (2008)
Norman Davies, God's Playground: A History of Poland, Vol. 2: 1795 to the Present, Revised Edition (2005)
Timothy Snyder, The Reconstruction of Nations: Poland, Ukraine, Lithuania, Belarus, 1569–1999 (2004)
Theodore R. Weeks (2006) "A Multi-ethnic City in Transition: Vilnius's Stormy Decade, 1939-1949", Eurasian Geography and Economics, 47:2, 153-175 (Apparently he has a book on Vilnius, Vilnius between Nations, 1795–2000 (2015), which I'm sure incorporates this article; however I haven't read it and cannot vouch for its reliability)
And I don't like to link directly Wikipedia for stuff, but the article on the 1920 Polish-Lithuanian War is pretty good, and includes Lithuanian-language sources, which I can't read at all.