Regarding the idea of Engels and Marx as a collaboratory pair, I’ve already discussed the lack of Collaboration in Capital, but here I will discuss their earlier collaborations. “German Ideology” will be saved for last because its context is the most complicated. “Holy Family” was Marx and Engels’ first collaboration. Engels was first author because he was better known at the time. He wrote his piece in it, before departing back to his family in the Wupper valley in Prussia. Marx remained in Paris and wrote his contribution, which formed the majority of the work. Because they wrote their parts separately, it’s different from a true collaboration imo.

The Communist Manifesto is closer to collaboration and unity of ideas. While Marx wrote it “himself” (as always he discussed it with Jenny, there’s some evidence that some of the phrases in the Manifesto are hers), it was based on the catechism Engels wrote beforehand. Imo its honestly not a great theoretical text, it can and should be criticised for a lotta stuff (eurocentrism, “imperialism is progressive” takes, linear stagist view of history (Marx was only 29 at the time, he got better)). The style is still amazing ofc.

The German Ideology Articles are more complicated. They were a collaborative work of self clarification by Marx and Engels, actually written together unlike the other two collaborations.This is the point in their lives when they were most on the same page (from here until Marx is in London imo). However there’s three other points which must be made. First, the articles were originally written not as one work, but as a series of articles dealing with various german philosophers in 1846. “The German Ideology” is the title it was given by the MEGA1 project. The editors of MEGA1 not only combined these articles into one “book,” they inserted into the text sentences and paragraphs Marx had written in sheets of contemporaneous notes and made inferences in some places where the text was incomplete. In justification they claimed they were illuminating the “dialectical coherence” of the text. (Johnson 2022) Further, when Engels rediscovered the manuscripts some time after Marx's death, he remarked that even "the finished portion [of Feuerbach]...proves only how incomplete our knowledge of economic hisotry still was at that time." (Engels, Ludwig Feuerbach, preface, 1886), there are an array of issues with it, not least of which its stagist linear view of history and belief that the steam engine caused capitalism (the latter of which is explicitly corrected by Marx in chapter 15 of Capital).

So Engels and Marx’s three [edit: after waking up and making another check of this I finally remembered Anti-Duhring Exists. Its writing was more in the vein of Holy Family as best as we can tell. Engels claim to have read the entire thing to Marx wasn't made until post-1883, however this should not be understood as "Engels was a lier," it likely simply wasn't really relevant to mention Marx's knowledge of it in writing until this point, he most likely agreed with at least the broad points.] works which were collaborations were written before either had turned 30, and before the vast, vast majority of marx’s research. This point— from Marx’s second meeting with engels in paris to his flight to london as a refugee — marks the most united the two men ever were in views as far as I can tell from my investigations. After the failure of the revolutions of 48, Marx found himself in desperate straits; half his kids died in the slums of london. Engels went back to his father and worked at the mill, this was the end of the period where marx and engels collaborated as partners in theoretical matters, and the beginning of Engels helping marx make rent and groceries (though marx was not a freeloader, he kept a steady job as a correspondent for the new york tribune and wrote other articles), he had much less free time to devote to theory during his working period. It seems natural to me that their perspectives would diverge to some extent.

So to summarise: engels differs from marx wrt theory in some respects — more teleological (in engels view history moves through fixed, discrete stages which will eventually lead to an “end” of history in a communism based on large scale industrial production and luxury consumption), less belief in the colonised as revolutionary subjects (in engels view they are objects of imperialism which will be transformed (cultural, etc, genocide) into revolutionary subject urban industrial proletarians), less multilinear/horizontal and more stagist/vertical view of history (in engels’ view, linear history, each stage an ‘advance’ on the previous leading to an ultimate end of history). Their collaboration in terms of theory was mostly a thing of their early lives, we should instead turn to Jenny Marx (sr.) and Eleanor Marx as Karl Marx’s most immediate theoretical collaborators.

Stalin differs from Engels in terms of theory, but they were similar (in a good way) imo in that they were able to take highly complex and abstract ideas and simplify and make them more applicable to the situation at hand. Deng was similar in this regard. The main theoretical difference as I understand it is that Engels believed in the revolutionary proletarian subject whereas the bolshevik analysis led to an anti-imperialist rural majority revolution with tenuous urban leadership (which solidified later).