Joining two words to make a single word

We call the "single words" you're talking about compound words.

It sounds like you're torn between creating closed and open compound words. The way to choose is to look at what other people are doing. The English language is always being reshaped by how we use it, so while "living room" might be two words today, who knows what will happen in 10 years!

In technical writing, readability always comes first. Choose whatever you think will make your writing more clear and understandable.

Compounding or composition is the process of word formation that creates compound lexemes. That is, in familiar terms, compounding occurs when two or more words are joined to make one longer word.
The meaning of the compound may be similar to or different from the meanings of its components in isolation. The component stems of a compound may be of the same part of speech—as in the case of the English word footpath, composed of the two nouns foot and path—or they may belong to different parts of speech, as in the case of the English word blackbird, composed of the adjective black and the noun bird.

Formation of compounds
Compound formation rules vary widely across language types. Compounds can be rather long when translating technical documents from English to some other language, since the lengths of the words are theoretically unlimited, especially in chemical terminology.

endocentric: A+B denotes a special kind of B
Eg: darkroom, smalltalk
exocentric: A+B denotes a special kind of an unexpressed semantic head
Eg: skinhead, paleface (head: 'person')
copulative: A+B denotes 'the sum' of what A and B denot
Eg: bittersweet, sleepwalk
appositional: A and B provide different descriptions for the same referent
Eg: actor-director, maidservant

It should be noted that there is "portmanteau", a piece of luggage that opens into two halves. This word has been "borrowed" (thanks to Lewis Carroll) to describe a word that is composed of parts of two other words, such as "motel" being derived from "motor hotel". https://en.wikipedia.org/wiki/Portmanteau