a bandpass filter that build up by two simple a RC-lowpass and a CR-highpass in series. When combines those two, the highpass affect the lowpass and vice versa. how to calculate the both cutoff frequency?

 

They combine to make a very poor filter because they are too simple. If the bandwidth is as narrow as is possible (the RC values are the same) then they attenuate everything including the frequency you want.

 

You should also use a operational amplifier between the two sections. The two sections need to be "isolated" from each other

 

You can do almost as well as using a buffer between the sections by scaling the impedances so that the second section doesn't load the first.

For example, if the first section consists of a 10k resistor and .01F cap, then make the second stage from a 100k resistor and .001μF cap.

Even then, it's not a very good filter. With only Rs and Cs, it's not possible to build a circuit with complex poles; all the poles are on the negative real axis, and good filters need complex poles (and zeros).