I try to create a related field on OpenERP 6.0.1 . is it possible to define two different onetomany relationfor the same field name? What all changes i must do in the(.py file and XML Files).
No you cannot do that: 

the field names are keys in a Python dictionary, in what you write the second invoice_line will overwrite the first one
this would mess up OpenERP's ORM anyway as it does not handle relations to different tables. 

So you need two different columns, one relative to account.invoice.line and the other to account.service.line. If you really need a merged view, then you can add a function field which will return the union of the invoice and service lines found by the two previous fields. But I'm not sure the forms will be able to handle this.
onetomany relation field in Openerp