I think that here it means a conjunction as in the astrological sense, i.e. when sometimes two or more planents are in the same general area of the sky seen from the earth -- even though for most of the year their different orbits and distances serves to place them in different areas.
Thus, referring to your context, while normally the author's sensitivity of observation and his writing style may not produce any marked combined effect, in some instances (perhaps when writing on certain themes/subjects) it does.
One can't say synergy here as this would suggest that the two effects always combined and I do not think that this may be the case.
Likewise with combination.
Perhaps coincidence may be closer to the idea.
Personally I favour the word conjunction (the literal translation) as I have seen it used often in historical (e.g. "conjunction of tendencies" bringing about historical change) or biographical writing (conjunction of external circumstances leading 2 otherwise disinterested people to form a partnership) and it is an effective metaphor for the process.