Depending on when this beta was built and how the output is configured I'd first start by saying that you should never hot swap or plug in headphones on a beta²². That's a good way to release the magic smoke and render the amp useless. Unless you're talking about an amp configured with a connector other than the usual 1/4" TRS jack. That jack causes a momentary short on the output when plugging/unplugging headphones that has been known to cause major issues.
That said, there should really be an issue powering two headphones at once off of the beta, just that if the sensitivities/load are significantly different it may be difficult or impossible to find a happy listening volume.