Most servos today are rated for up to 6V or more, but operate fine at 4.8V. I have only run leads 3M or so, but agree 20M should work. If voltage drop is a problem, you could increase your voltage. These servos can draw a surprising amount of current under load, enough so that loading one might cause the other to twitch from low voltage.
As for the twitching, how close are the leads to each other? Are you running 6 lines (i.e., 2 power for each servo plus 2 signal) or only 4 lines (i.e., share the power lines)? Please confirm that there is no twitching with only one servo. Many cheap controllers, particularly the 555-based ones, cause twitching. In models, we routinely twist the leads, but I have no data to support that practice.
John