• sutalam — Sutala (a region of the lower planetary system)
  One of the seven sub-terrestrial regions. Bali Mahārāja was given sovereignty of Sutala by Vāmana-deva after surrendering everything (SB 8.22). It is the abode of a great devotee, not a hellish region.
• yoga-māyāyam upāśritau — taking shelter of Yoga-māyā
  Compound + participle: yoga-māyā + upa + ā + √śri (to take shelter of). The Lord moves between regions not by physical travel but by His own internal potency. Yoga-māyā is His private vehicle.
• saṁviviśatuḥ — the two of Them entered
  Dual aorist of sam + vi + √viś, 'to enter together'. Both brothers enter as one act. Gauḍīyas: where Balarāma goes, Kṛṣṇa goes; their movements are coordinated tattva.
• sañcoditau — thus urged
  Dual past participle of sam + √cud, 'to be urged'. Kṛṣṇa moves only when prompted by His devotee's prayer. Devakī's vaiklavya is the actual cause of the Sutala-journey.

Two brothers step into a hidden chamber the way you might step into a memory — the door is yoga-māyā, the chamber is sutala, and what waits inside is a king who has been ready for this visit for ages. Their movement is not travel; it is the rearrangement of where they are.