First, PLZ realize that there are huge, very important differences between correlation and cause-and-effect. Correlation simply means that two variables rise/fall in a predictable way together. The cause-and-effect relationship is one where the change in one variable directly effects (note: not "affects") the corresponding change in another variable.
For example (PLZ read Wikipedia article on SAT), colleges grouped together to form the Scolastic Aptitude Test to screen applicants to choose the most likely to graduate (and contribute as alumni). Now just SAT, the test is still used to screen applications and to determine scholarships, etc., but PLZ realize that a high/low score on the SAT test does not produce a college degree (actual course work does). So, although there is a correlation, more work is needed to determine the variables involved in cause-and-effect.
In jest, a person said, "If you ever find the perfect church, don't join it." [it will no longer be perfect.] A similar argument goes for "students who eat breakfast perform better in school" [so we feed bad students breakfast year round, which lessen the correlation of "breakfast" to "better performance in school."]
So, you must first decide whether having or not having a two-party political system effects [again, not "affects"] high/low voter turnout. Or, are there lots of hidden variables that do this?
Now, although political candidates may offer a choice between very bad and much worse, at least we have the precious privilege of voting. Many countries allow citizens to "vote" for the one candidate running for office. Low turnout happens when  the voting system doesn't allow easy access to the polls (in America, early voting has greatly eased this),  one candidate (perhaps an incumbent) is successful and highly liked over all others (so people think that their vote doesn't make a difference),  early information about voting results indicates winners and losers [note: if you live in Hawaii, you might not vote based on the outcome already known], and other factors. High voter turnout is more likely when there are hotly contested issues [for example, proposed local zoning laws get very high voter turnout] or candidates represent very different political views. Although not a political party example, there is quite high participation when union members vote whether to strike (interest is high because they don't get paid while on strike).
Once you have presented your argument, be sure that changes to the variables you indicated (here, it is whether or not there is a two-party system or whether there is an effective two-party system or whether there is a balanced two-party system or ...) actually, directly cause high/low voter turnout -- or, maybe, other things do.