We don't have a two party system.
We have two more or less coalitions containing at least two parties each, with one coalition being vastly more powerful based on sheer numbers and demographics.
Let's be honest, the Peace party is at this point a puppet of the Labor party (a merger got discussed and approved by Labor in fact, but Peace never got around to voting it). The ACP is only marginally more independent from the Federalist Party. So we do have a "de facto" two party system; in a way not too different from how Australia has the Liberal/National coalition (except in Atlasia it's both sides that do it).
I would also disagree with the notion that the left is somehow advantaged in elections. Even if we stick just with the last year's resurrection of Labor and death of PUP, the presidency has been no better than a tossup (and if anything, with an extremely slight right wing lean though that may be due to candidates).
Both October 2018 and June 2019 were razor thin victories for WB and Griffin (2 and 3 points respectively); while February 2019 was a more comfortable victory for tmth (7 points if I remember correctly).
The House, while having an slightly larger left wing advantage (possibly since it's fought more on policy and personality matters less?) is definitely winnable for the right, as February proved. June was also not exactly a run-away victory (7 points in the 2 party preferred, comparable to tmth's presidential win)
I will admit the Senate (and by extension the governorships/legislatures) is hard to win for the right, but not impossible. The left wing advantage is fragile, only requiring an upset in Fremont or possibly Lincoln (the current situation of 5-1 is an anomaly).
And that's just sticking to the last year (which has been good for the left overall, holding a trifecta for 6 of the last 10 months and with a trifecta for the next session of Congress as well). I would remind that before WB's win, there had been no left wing outright victories since pre-reset. Similarly, the House had oscillated between left and right wing control (and no overall control), for the previous year.