Can two large business Prime IDIQ holders form a joint venture for a task order under that IDIQ?

I assume that you are not asking about a GSA Federal Supply Schedule contract.

If the joint venture is not itself one of the contractors, then the answer is no, because a CO cannot issue an order to an entity that is not an awardee. But the parties can team up, with the order being issued to one or the other of them.

If the agency's rules and the terms of the contract permit, the parties might be able to get the agency to award an order jointly to both of the them, with the parties being jointly liable. But that would be complicated. As a CO, I probably would not do it.

I suggest seeking legal counsel on the idea whether you are on the Government or private sector side.   The idea creates, as already noted,  many considerations/complications with regard to the IDIQ and task order itself as well as internally to each entity.   With regard to contract/task order processes  the questions that quickly come to mind are - Is the JV a successor in interest (Reference FAR subpart 42.12)?; Does the JV need to become registered in SAM: Does the JV need its own TIN; How would award be handled through the electronic data systems now used by many agencies?;  What happens with regard to the subcontracting plans that each of the members to the JV might already hold?;  along with many other questions I suspect.,  etc.