Japanese firebelly newt with Chinese Firebelly newt?

Can the two species safely be housed together?

It's not a good idea - Japanese firebellies are larger so the smaller newt might get bullied. It's generally best to have one species per tank, and not just between newts. There are issues like toxicity, differences in tank requirements, possibility of inappropriate mating attempts, and general bullying which might result in poor feeding, high stress levels etc. if you house different animals together.
Have you read these articles?
This one deals with species mixing: http://www.caudata.org/cc/articles/Mixing_disasters.shtml

Yes I did read those articles but they had no examples of the two species I mentioned so I figured they might be compatable, especially since they are so closely related, similar, Japanese newts are only slightly bigger.
But I'm not going to mix I was just wondering if anyone did and if they hybridize.

They aren´t that closely related, but they are capable of producing hybrids, which is a concern.
The two species are quite unagressive, but like Eva, said, Cynops pyrrhogaster gets substantially larger than Hypselotriton orientalis, and they tend to be food hoovers, so they could easily outcompete the smaller newts. In the event of any aggression, the superior size and strength of pyrrhogaster could mean serious damage. Even a bite in the confussion of a feeding frenzy could result in a damaged or missing limb. Not a likely event, but entirely possible.
For me the main point is not possible harm in this case, but rather the complete pointlessness of assuming any risks by housing them together. What on earth is the benefit of that, really? And how does that hypothetical benefit warrant assuming any risks (agression, pathogens, hybridisation, stress)?