can i do like mount ethernet shiled to arduino and mount another bluetooth shield? and mix up codes for both bluetooth and ethernet??
You do not say what shields you have but it is normal for them to be stackable but you need to look out for the boards and associated software expecting to use the same pin numbers as one another. For instance, if one shield uses pin 10 as input and another uses it as an output then neither will work.
There is also the possibility that both shields might need to use some facility of the Arduino, typically timers, but they can't both use them at the same time. Assuming that there is no conflict with the use of pins and other resources, then, yes, you can use both concurrently and write code for both in the same sketch.
You may get more specific advice if you post details of your Arduino and the shields.