There were two altars in the Beis Hamikdash. The Mizbeach Hachitzon, the outer altar used for the sacrifices, stood in the courtyard. Inside the Heichal was the Mizbeach Hapenimi, the inner altar which was used for offering Ketores.
The Alter Rebbe identifies two levels of emotional connection, two experiences of fiery love for Hashem. These two dimensions are represented by the two altars.
The outer altar represents the emotions of the conscious soul, referred to as חיצוניות הלב, the external heart. These are the feelings of love for Hashem that are developed through meditation on Hashem’s greatness.
The inner heart (פנימיות הלב), represented by the inner altar, is the essential desire which comes from the essence of the soul. This desire transcends reason and understanding. It is the implicit, powerful desire for Hashem alone that every Jew possesses.
The correlation reflects itself in the Avodah of each of the two altars.
Korban (sacrifice) comes from the word Karov, drawing close. The fire generated by meditation draws us close to Hashem. The fires of the outer altar, consume the desires of the animalistic soul. But we remain separate and the experience dissipates.
Ketores (incense) is related to the Aramaic term for a knot. A knot binds two things together so that they become one. This is the suprarational love and desire from the essence. Once revealed, this love never passes.
Tanya Igeres Hakodesh chapter 4
Likkutei Torah Sukkos, Ushavtem Mayim
Hemshech Ayin Beis chapter 212