Why isn't there any repulsive force between Na+ and K+ disrupting thier roles in transmembrane voltage/ action potentials
The carriers of the charge are ions and they get repelled from each other well enough. Other than their charge there is only the size in which they differ (for all practical purposes). This means, as long as we are talking about membrane potential, the actors are just a mix of 1+ ions which don't come near each other. When size matters, for example in an ion channel, we are talking about single particles, so there is no action of a repulsive force between K+ and Na+.

From another angle: The charge is a force that emerges from a sea of particles, regardless if they are all of one kind or mixed K+/Na+. If you talk about a force between K+ and Na+ then you would have to separate all 10^15 or so K+ ions from 10^15 or so Na+ ions which is simply impossible, as they are mixed up.

http://en.wikipedia.org/wiki/Brownian_motion

http://en.wikipedia.org/wiki/Ion

http://en.wikipedia.org/wiki/Ion_channel