Proportional Control Balance Beam
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Proportional and "anti-proportional" gain control where both motors sum to unity.

I'm no expert but I would be doing something like this:

// e is the error, Kp is proportional gain
MotorA = 0.5 + Kp * e         // 50% power at zero error
if (MotorA &lt; 0) {MotorA = 0}  // Don't go below 0%
if (MotorA &gt; 1) {MotorA = 1}  // Don't go above 100%
MotorB = 1 - MotorA           // Sum to 100%


At balance the motors will run at 50%.



Update after revised code added:

float error = sqrt(roll*roll);
Positive_esc = 0.15 + (error*Kp);
Negative_esc = 0.3 - Positive_esc;

if(roll&lt;0){
  LeftMotor = Positive_esc;
  RightMotor = Negative_esc;
}
if(roll&gt;0){
  LeftMotor = Negative_esc;
  RightMotor = Positive_esc;
}


This code gives the following transfer functions.







simulate this circuit

Figure 2. The upgraded code has major issues.

Note that your Positive_esc and Negative_esc are not complimentary. The big problem is the major discontinuity of the left and right motor drives around the zero point.

Did you try my suggested code?