The idea that Indrāgñi is a combination of Indra and Agñi is something that we reach for when our knowledge of Sanskrit and the Veda is not deep enough.
So there is a linguistic problem and a “lore” problem. Here is some detail on the linguistic problem:
The yogikārtha (denotational meaning) of “Indra” is “defeater of others” aka “best.” There is not yoga-rudhikārtha (exclusive connotation) for the word. You can see it is regularly used with its yogikārtha (denotation) in very common names like Mahendra, Dharmendra, Mohendra, etc. etc. (In all these names, the principles of sandhi [phonetic convergence] cause the initial “i” of Indra to change to “e” due to meeting with “a” at the end of the previous syllable).
Using Indra as a name for the Cheif God, defeater of demons, is a rudhikārtha (nonexclusive connotation). Therefore there is no grounds for claiming that Indrāgñi is a samāsa (compound) of two proper nouns.
Similarly, agñi denotes fire, and is used as a proper noun for a devata by the same nonexclusive connotation principle (ruddhikārtha). Actually almost all of the devatā’s (Vāyu for example) are named in this way (by nonexclusive connotation of the noun that describes the domain of reality over which they preside).
Now, for the “lore” problem.
There is an entity called Indrāgñī. You will find him mentioned in Ṛg and Mahābharata. He is “the best (indra) fire (agñi),” i.e. the fire of consciousness / comprehension. “The fire in the heart / mind.”
The second best form of fire is Agnīsoma, this is “The fire of health” – again not a combination of two devatas (Agni and Soma).
Both Indrāgñī and Agnīsoma are found in their personified forms on the right and left of Brahmā, the creator. By their presence they ensure that Brahmā has the maximum intellect and lifespan.
So, both linguistically and by lore, it is a mistake to say that “Indrāgñī” is a compound of proper nouns – and thus a dual god.
What is even more interesting than this, to me at least, is that another meaning of agñī is śakti. Therefore the word Indrāgñī is synonymous with Indra-śakti, who is his consort, Indrāṇī. I personally consider Indrāṇī Śacī Devī as the Devī of Viśākhā, as I explain in the video (https://www.youtube.com/watch?v=_SlBsMc16mQ)