how to synchronize two flexfields on the two Oracle built-in form?

I have not known that those forms cannot be edited directly. If you can edit them then a parameter can be added to the called form. In a calling form the parameter can be set and pass to the called form.

I think that in such situation you can add an item (as a package variable) to the custom.pll. In calling form this item can be set and in called form it can be read. But I don't know how it can be done if forms cannot be edited and custom.pll is only attached to them.

If you do not have access to edit the forms, then you have a tedious job of finding some existing logic in the forms which can help you in a tricky work-around.

Let us break down your problem into two.
First: You would not be able to pass values to child form unless you have a option of getting the master flexfield value outside the master form. So first step is to find a solution to get flexfield value outside.
Second: To search for some methods to pass values to child form.

Let us tackle the first problem:

If you have read access to the form, then try the following steps:
1. If the master flexfield LOV returns the selected values to some global variable or package variable, then you have an option of getting the selected value outside the master form.
2. If the master form has any suitable trigger (like when-button-pressed or post-form) which is subclassed to custom.pll, then you could have an option to get the selected flexfield value.
3. If the master form has any other method of passing selected flexfield values outside its scope, like passing parameter outside the form, inserting selected data to a temporary table, etc.

If you are able to get the selected flexfield value outside the master form, then start with the second task.
1. Check, how is the child form called? If it is called by passing parameters for flexfield, then you need to pass the master form flexfield value to it.
2. Check if the child form has any form initializing triggers (like when-new-form-instance, when-new-item-instance, etc) which is subclassed to custom.pll. This can help you in initializing flexfield value at the opening of the form.

Finally I would say that the availability of custom.pll for editing would not be of any help until your have a some possibility of fetching and passing values from master to child is already in place.

I have read the document and I think that you can do it.