What is it called when a speaker accidentally merges two words?

Solution 1:

lexical/word blend errors

Word blend errors should be examined for evidence to support this hypothesis, but the facts in the MIT-CU corpus are obscured by the number of cases that blend at a shared phoneme (e.g., "prubble" for [problem + trouble]), making it impossible to determine whether the blended portions correspond to the initial consonant sequence and the rest of the word or not. Pyscholinguistics: Critical Concepts in Psychology ref.

In paradigmatic lexical blend errors, the most common semantic relationship for the adults was synonyms...The frequency of synonyms is what is expected, given the definition of lexical blends: if two lexical items are competing for the same slot in the utterance, then the most likely situation will be that the two words will convey nearly the same meaning. Kids' Slips ref.

While speech errors come in many forms... Semantic errors are of two types, word substitutions and word blends.
...
As discussed in 2.3.4, substitution and blend errors frequently involve words that are semantically related in a paradigmatic way. This leads many to the conclusion that the mapping errors involved here arise from the "proximity" of paradigmatically related words in a semantically organized lexicon. Semantic Relations and the Lexicon: Antonymy, Synonymy, and Other Paradigms (2003) ref

word blend errors is an index entry in this last ref.

Solution 2:

In psycholinguistics, that might be referred to as a speech error — a “blend” of the unintentional type. For example:

Blend “errors” involve two different lexical units that are planned for the same slot in a phrase and their phonological forms blend together in a single unit:

(11) That’s a great big [fɑpɑ] bear! (father and papa) (Jaeger 2005)

This example from a child resulted in a blend of the words father and papa, two related lexical items planned for that position in the sentence.

_{Source: To “Err” is Human: The Nature of Phonological “Errors” in Language Development}

Portmanteaus are blends too, but they are intentional.

That’s the best I can do. But before I go, I thought I would offer a word I accidentally invented years ago — in a nomenclature meta moment:

termanalogy n.

1. An improvised word similar to the one you were aiming to say.