Yes. There is nothing that says only one leader stacked with can excute such a rout... To form the restriction you envision the rule would need to read "Only one non-berserk, non-pinned leader already stacked... " (Eeeegad, you need some time on the couch Specialist Pyth, I worry now about this compounded by the 5/8" Dummy Jeep jitters...)
A non-berserk, non-pinned leader already stacked with a broken unit before it routs may elect to rout with the broken unit even though he is not broken. If he does so, the leader shares the broken unit’s vulnerability to Interdiction and, although he does not have to take any Interdiction NMC himself, he is eliminated if the broken unit he is stacked on top of fails an Interdiction MC. He must remain with the broken unit throughout the RtPh, but is not considered broken and may add his leadership DRM to its Interdiction NMC. The leader, if already in possession of a SW, may portage it, but cannot improve the broken unit’s portage capacity.