Receiving two values over serial port using servos
Your problem is that you aren't providing anything to split the data on. You just send the X and y coordinates with a : between X and y. There's nothing between the y and the next X.

By changing the line

port.write(x+":"+y);


To

port.write(x+":"+y+"\n");


You then get the linefeed character to split your data on.