How can I send/write two doubles via Bluetooth (nRF8001 Bluefruit LE) in Arduino?
As the link "Adafruit - nRF UART In Detail" describes, the function write(sendbuffer, sendbuffersize); is waiting for a buffer of uint8_t followed by the size of the message.

Here are the step--by-step solution to send the 2 doubles by using that function.

Step1 - convert the floating-point values into a char string.


  In the Arduino environment, the standard format-specifier %f or %lf
  is not well-managed and the function dtostrf() is recommended.


The format is char *   dtostrf (double __val, signed char __width, unsigned char __prec, char *__s) where:


__val is the floating-point value to be converted,
__s is the array of char to store the result,
__width is the number of characters to store the result,
__prec is the number of digit


The conversion is:

char sValueOne[16], sValueTwo[16];
char sBuffer[33];

strcpy(sBuffer,dtostrf(valueOne, 15, 8, sValueOne));
strcat(sBuffer,",");
strcat(sBuffer,dtostrf(valueTwo, 15, 8, sValueTwo));


Step2 - write the buffer over the BTLEserial object.

BTLEserial.write((uint8_t *)sBuffer, strlen(sBuffer));



  Alternate output solution using BTLEserial.print() function.


BTLEserial.print(sBuffer);