Kṛṣṇa is not speaking about a variety of paths leading to a variety of inferior or superior destinations. Rather, He is describing two aspects of a path - one easy and one difficult-with the same goal. Only the ignorant speak of karma yoga and Sāṅkhya yoga as different thinking that they have different fruits. He who applies himself well to one of these paths attains the result of both ¬- seeing the ātmā. One who sees that the goal of jñāna can also be attained by karma yoga, sees things as they are. The steady position of seeing the ātmā, which is attained by the jñāna-yogīs is also attained by the karma yogīs , though one is renunciation of action and the other being engaged in action.
The word sāṅkhya refers to being fixed in jñāna. An aṅga of that is sannyāsa. Only the fools speak of the difference between sannyāsa and karma yoga. Kṛṣṇa has already stated that the karma yogī is a real sannyāsī. Thus these two are aspects of the same path, with the same goal. Karma yoga done with detachment allows us to become materially detached and spiritually attached (realizing the soul) at the same time.
If we understand karma yoga to mean actions done for Kṛṣṇa, then the unity of karma yoga and Sāṅkhya yoga can be understood as follows: The aim of the analytical study (Sāṅkhya) of the material world is to find the soul of existence. The soul of the material world is Viṣṇu, or the Supersoul. Devotional service to the Lord entails service to the Supersoul. One process (Sāṅkhya or jñāna) is to find the root of the tree, and the other (karma) is to water the root. The real student of Sāṅkhya philosophy finds the root of the material world, Viṣṇu, and then, in perfect knowledge, engages himself in the service of the Lord.
By Sāṅkhya philosophical research one comes to the conclusion that a living entity is not a part and parcel of the material world but of the supreme spirit whole. Consequently, the spirit soul has nothing to do with the material world; his actions must be in some relation with the Supreme. When he acts in Kṛṣṇa consciousness (karma yoga), he is actually in his constitutional position. In the first process, Sāṅkhya, one has to become detached from matter, and in the devotional yoga process one has to attach himself to the work of Kṛṣṇa consciousness. Therefore, in essence, there is no difference between the two because the aim of both is Viṣṇu.