The Visistadvaita interpretation of the phrase "one only without a second" is that Sriman Narayana is unique and no other being is equal to him, not that no other being exists. Here is what Yamunacharya, who was Ramanujacharya's guru's guru, says in his Siddhitrayam:
To say that the Chola king, now reigning in this country, is all supreme and without a second, can only exclude the existence of another monarch equal
(in power) to him; it cannot imply the denial of the existence of a wife, sons or servants of such a monarch.
Ramanujacharya says the same thing in this section of the Sri Bhashya, his commentary on the Brahma Sutras:
What the phrase 'without a second' really aims at intimating is that Brahman possesses manifold powers, and this it does by denying the existence of another ruling principle different from Brahman. That Brahman actually possesses manifold powers the text shows further on, 'It thought, may I be many, may I grow forth,' and 'it sent forth fire,' and so on.--But how are we to know that the mere phrase 'without a second' is meant to negative the existence of all other causes in general?--As follows, we reply. The clause 'Being only this was in the beginning, one only,' teaches that Brahman when about to create constitutes the substantial cause of the world. Here the idea of some further operative cause capable of giving rise to the effect naturally presents itself to the mind, and hence we understand that the added clause 'without a second' is meant to negative such an additional cause. If it were meant absolutely to deny all duality, it would deny also the eternity and other attributes of Brahman which you yourself assume.
So in Ramanujacharya's view, that phrase indicates that Brahman is the only efficient cause and material cause of the Universe - there is no second cause. This is elaborated in another section of the Sri Bhashya:
This clearly shows the unity of the operative (ruling or supervising) cause and the material cause; taken in conjunction with the subsequent declaration of the unity of the cause previous to creation, 'Being only, this was in the beginning, one only,' and the denial of a further operative cause implied in the further qualification 'advitîyam,' i.e. 'without a second.'--But how then have we to understand texts such as the one quoted above (from the Kûlika-Upanishad) which declare Prakriti to be eternal and the material cause of the world?--Prakriti, we reply, in such passages denotes Brahman in its causal phase when names and forms are not yet distinguished. For a principle independent of Brahman does not exist, as we know from texts such as 'Everything abandons him who views anything as apart from the Self; and 'But where for him the Self has become all, whereby should he see whom?' (Bri. Up. II, 4, 6; 15). Consider also the texts, 'All this is Brahman' (Kh. Up. III, 14, 1); and 'All this has its Self in that' (Kh. Up. VI, 8, 7); which declare that the world whether in its causal or its effected condition has Brahman for its Self. The relation of the world to Brahman has to be conceived in agreement with scriptural texts such as 'He who moves within the earth,' &c., up to 'He who moves within the Imperishable'; and 'He who dwells within the earth,' &c., up to 'He who dwells within the Self (Bri. Up. III, 7, 3-23). The highest Brahman, having the whole aggregate of non-sentient and sentient beings for its body, ever is the Self of all. Sometimes, however, names and forms are not evolved, not distinguished in Brahman; at other times they are evolved, distinct. In the latter state Brahman is called an effect and manifold; in the former it is called one, without a second, the cause. This causal state of Brahman is meant where the text quoted above speaks of the cow without beginning and end, giving birth to effects, and so on.