I want to write two a5 pages in the same a4 page (in landscape format). Is
it possible? I'm workinf woth jakarta FOP API.

You seem to ask for some sort of "print two document pages on
one sheet of paper". If so, whether it is possible depends
heavily on how you have to print it. The easiest way is
still to format the document on a A5 page master and use a
some printing tool to tell the printer to do what you want.

In XSLFO there is not really an easy way to get two pages
onto one page master, by definition. Depending on your
document, in can be emulated in certain circumstances.