SB 10.46.31

etau hi viśvasya ca bīja-yonī
 rāmo mukundaḥ puruṣaḥ pradhānam
anvīya bhūteṣu vilakṣaṇasya
 jñānasya ceśāta imau purāṇau
Word for word: 
etau — these two; hi — indeed; viśvasya — of the universe; ca — and; bīja — the seed; yonī — and the womb; rāmaḥ — Lord Balarāma; mukundaḥ — Lord Kṛṣṇa; puruṣaḥ — the creating Lord; pradhānam — His creative energy; anvīya — entering; bhūteṣu — within all living beings; vilakṣaṇasya — confused or perceiving; jñānasya — knowledge; ca — and; īśāte — control; imau — They; purāṇau — primeval.
Translation: 
These two Lords, Mukunda and Balarāma, are each the seed and womb of the universe, the creator and His creative potency. They enter the hearts of living beings and control their conditioned awareness. They are the primeval Supreme.
Purport: 

The word vilakṣaṇa means either “distinctly perceiving” or “confused,” depending on how the prefix vi is understood in context. In the case of enlightened souls, vilakṣaṇa means “perceiving the correct distinction between the body and the soul” and thus Lord Kṛṣṇa, as indicated by the word īśāte, guides the spiritually advancing soul. The other meaning of vilakṣaṇa — “confused” or “bewildered” — clearly applies to those who have not understood the difference between the soul and the body, or the distinction between the individual soul and the Supreme Soul. Such bewildered living beings do not go back home, back to Godhead, to the eternal spiritual world, but rather achieve temporary destinations according to the laws of nature.

It is understood from all Vaiṣṇava literature that Śrī Rāma, Balarāma, here accompanying Lord Kṛṣṇa, is nondifferent from Him, being His plenary expansion. The Lord is one, yet He expands Himself, and thus Lord Balarāma in no way compromises the principle of monotheism.