Unless you're doing a fairly rapid roll, you will need to learn the use of rudder during the roll. 2 seconds is a relatively slow roll. The AMA slow roll definition is 3 seconds minimum and you definitely need to integrate rudder into it and not just while it's knife edge. You will find that to hold a line during a slow roll, you need constantly varying elevator and rudder inputs to not only hold altitude, but to counter each other's effects that individually will take the model off the straight line you want.

So for example, you're rolling right. as the model continues to bank more, you will naturally want to feed in some up elevator, but that is going to start a turn to the right. You have to counteract that by putting in a corresponding left rudder input that increases as the model approaches knife edge, while the up input begins to decrease, becoming zero at knife edge. The same idea applies all around the entire roll; you will have to think ahead to visualize what the required inputs are going to be. This may take a lot of practice!

You can definitely start by getting the basic mechanics down by just assuring that you knock the correct direction of rudder in, even if you see a wobbly result at first. You will at least train yourself and develop some muscle memory so that you don't constantly question yourself about what the basic rudder input is in the knife edge quadrants. Then you start working on the finesse of dialing in more rudder as elevator input increases and decreases as the roll progresses towards 90 degrees and 270 degrees. BTW, it doesn't take a whole lot of rudder/elevator when this is done right and you get them working together effectively to maintain altitude while cancelling each other horizontally.