the silverstripe database structure won't allow more than one parent id for a page, but there are other possibilities:
- use 'VirtualPage' in the sitetree (i was having problems with this one in previous silverstripe versions, can't say wheter it's doing fine now)
- refactor your code using dataobjects (see the tutorials on this or just post here again in case you need further advice)
hth

i suppose you're using 'category' pages, such as 'food' and 'fruit'.
so for a page called 'apple', you might first create the page in the sitetree as a child of 'food', then you could switch to 'fruit' and create a virtualpage there, pointing to the 'apple' page created under 'food'.

as for the dataobject approach, you should work through tutorial 5, but make sure you're already familiar with the concepts explained in the previous tutorials (except tutorial 4 maybe).