If I eat a pastry and grapes at one sitting, do i make two brachos achronot al hamichya and an al haetz? or may I combine the words into one bracha acharona by saying "al haetz V'al hamichya" ?
The latter is correct. Assuming you ate a proper shiur of both the pastry and the grapes (which creates a requirement for a bracha achrona), you would combine the bracha achrona.
However, the correct order would be "al hamichya v'al hakalkalah, v'al haetz v'al pri haetz" (על המחיה ועל הכלכלה, ועל העץ ועל פרי העץ)
In general, the correct order is:
Similarly, at the end you would substitute the for all the food-types you ate...
Source: Siddur Tefillas Shlomo (Artscroll), and Sefer Sharei Haberacha (page 311)