This synoptic pericope contains three distinct sayings:

• a warning to be alert,

• an aphorism concerning equal measures (give/get), &

• a riddle about possessions (have/given//have not/taken).

Mark is the only gospel that links all three at this point in the narrative.  Matthew had already used the third saying to explain Jesus' declaration that his disciples had been given "the secrets of the kingdom of heaven" while others had not [Matt 13:11].  Well before that he had linked the saying about equal measures to a warning about judging others in Jesus' Sermon on the Mount [Matt 7:2]. So, he does not present either these sayings or even the introductory call to alertness in a context parallel to Mark's. Yet, Matthew does duplicate the riddle about haves & have-nots much later in his narrative [Matt 25], as an appendix to the parable of entrusted funds. 

Luke, on the other hand, has the initial warning & the concluding conundrum in precisely the same sequence of sayings as Mark, right after the aphorisms of the lamp & the disclosed secrets, which Matthew records at different points in his narrative.  The one notable difference between this string of aphorisms in Mark & Luke is that Luke's version does not include the saying about equal measures. Rather, Luke presented that aphorism earlier [Luke 6] in a context roughly parallel to Matthew's in a shorter version of Jesus' sermon that Luke locates on a plain; so he does not repeat it here. Yet, like Matthew, Luke repeats the riddle about haves & have-nots at the conclusion of his version of the parable of entrusted funds [Luke 19].

Thus, with the exception of the saying about equal measures, Luke has close parallels to both Mark's string of aphorisms & two of the unstrung aphorisms that Matthew tied to other sayings.

Though Mark uses the inferential conjunction "for" to join the riddle of the haves & have-nots to the aphorism of equal measures, the relation of the logic of the second saying to the first is not clear.  In fact, in English translation these aphoristic observations seem mutually contradictory. For if only those who have can expect to get (second premise), there is no incentive to give anything away.  But if one gets as much as one gives (first premise), then those who have given all they have away can still expect to receive as much in return, contrary to the claim of the second aphorism.  Thus, the second saying appears to contradict the first & the first to undermine the second.

The first aphorism, however, is really about measuring rather than giving.  A more literal translation of the Greek would be: "in the measure you measure <out>, it will be measured <out> to you."  The reference is to some kind of trade. For the Greek word μέτρον means a rule or standard.  The point of saying "the standard you use will be used on you" in a world before the establishment of a universal metric system is to convince people to use fair or even generous standards rather than to try to cheat others.   

Unlike the saying about equal measures, the second aphorism encourages accumulation rather than fair transactions. Note that the nature of what one has (or has not) is left unspecified. Thus, the audience is left to discover a situation that fits the pronouncement. The context in Mark & Luke's parallel implies the possession (or lack) of knowledge or understanding.  But the saying can just as well be used in an economic setting where the possession is money. This is the connotation it acquires when appended to the parable of the entrusted funds later in Matthew & Luke [Matt 25:29]. In the latter case this aphorism becomes the equivalent of the English proverb: "the rich get richer and the poor get poorer."  That may be a common opinion of secular types. But in a Jewish (or Christian) culture that demands social justice such an observation becomes problematic. And it is even more of a conundrum when it is presented as a pronouncement of Jesus, whom the synoptics elsewhere represent as counseling a rich man to sell all his possessions & give the proceeds to the poor [Mark 10:17-22 & parallels].

In any case, it should be clear that these sayings were not formulated as one continuous line of reasoning.  Like most proverbs they are generalizations based on quite independent observations. Their conjunction in Mark is obviously secondary & artificial. Since Mark is the only text in which they are linked, these aphorisms probably had separate origins & circulated independently until Mark brought them together. Moreover, Mark's rationale for linking them is not clear. For there is no catchword common to both sayings in the original Greek that could explain why he recalled such unrelated observations in tandem.