Sensor Interference

Hi, I am working with TSL235 light to frequency converter sensors using an Arduino MEGA ADK. When working with one sensor at a time there is no problem with sensing. When two sensors are simultaneously connected they interfere with each other. How can i fix this problem ?, I am attaching the code being used for the simultaneous reading of two TSL235 sensors. Regards. Alejandro.

define TSL235Two 21 // Out of TSL235 connected to Digital pin 21

define TSL235One 3 // Out of TSL237 connected to Digital pin 3

int period = 101; // Miliseconds of each light frecuency measurement

unsigned long counterTwo = 0; // Counter of measurements during the test
unsigned long counterOne = 0; // Counter of measurements during the test

unsigned long currentTime = millis();
unsigned long startTime = currentTime;

volatile long pulsesTwo = 0; // Counter of measurements of the TSL235R
volatile long pulsesOne = 0; // Counter of measurements of the TSL235R

unsigned long frequencyTwo; // Read the frequency from the digital pin (pulses/second)
unsigned long frequencyOne; // Read the frequency from the digital pin (pulses/second)

void setup()
{

Serial.begin(115200); // Start and configure the serial port
attachInterrupt(2, PulseCountTwo, RISING);
attachInterrupt(1, PulseCountOne, RISING);
pinMode(TSL235Two, INPUT); // Declare the pin such as an input of data
pinMode(TSL235One, INPUT); // Declare the pin such as an input of data
setup_parallax();
}

void loop()
{

counterTwo++; // Increase the number of measurement
counterOne++; // Increase the number of measurement
getfrequencyTwo(); // Request to measure the frequency
getfrequencyOne(); // Request to measure the frequency
pulsesTwo = 0; // reset the pulses counter
pulsesOne = 0; // reset the pulses counter
parallax_output();

}

void PulseCountTwo()
{
pulsesTwo++;
}

void PulseCountOne()
{
pulsesOne++;
}

void parallax_output()
{

Serial.print("DATA,DATE,TIME,");
Serial.print(frequencyTwo); Serial.print(","); Serial.println(frequencyOne);

}

void setup_parallax()
{
Serial.println("LABEL,Date,Time,Frequency Two, Frequency One");
}

unsigned long getfrequencyOne ()
{
noInterrupts();
frequencyOne = pulsesOne *1000 / period; // Calculate the frequency (pulses/second)
interrupts();
return (frequencyOne);
}

unsigned long getfrequencyTwo ()
{
noInterrupts();
frequencyTwo = pulsesTwo *1000 / period; // Calculate the frequency (pulses/second)
interrupts();
return (frequencyTwo);
}

# define TSL235Two 21 // Out of TSL235 connected to Digital pin 21
# define TSL235One 3 // Out of TSL237 connected to Digital pin 3

attachInterrupt(2, PulseCountTwo, RISING);
attachInterrupt(1, PulseCountOne, RISING);

I am pretty noob but are those pins (21 and 3) then interrupt pins 2 and 1?

Ah sorry, they are. Ignore me.

Post your working code for testing a single sensor so we can compare. Your current code for two sensors does not seem to allow any time for the ISR to count up before reading the value and resetting it.
A quick search shows this thread may be of interest (especially reply #5)