If I want to make two parallel calls (in one flow activity) to the same web service in BPEL, do I need to create two different PartnerLinks or can I use the same one?
You can invoke the same partnerLink multiple times in parallel in the same flow. You do not need to define a new one.
If you expect a result from the invocation, you need different outputVariables, however. If you write to the same variable in parallel, you will have race conditions just like in any other programming language.