Pet-shop bought Cynops orientalis are all WC and adult/subadult.
As long as there isn´t a big difference in size, there shouldn´t be a problem. Even with much smaller juveniles, they are not an agressive species.
Before you even think about putting them together, though, remember to quarantine the new newt. Quarantine is very important!!
I also would like to say that land areas are a waste of space for this species. Adults can be kept 100% aquatic year-round with just a piece of floating cork bark as an emergency terrestrial area.
I suspect that if you have a big land area, and being a 10gallon which is pretty much the minimum volume recomended, the water area must be small. That´s bad cause small water volumes polute easily and are susceptible to temperature changes. For the proper caring of the two newts you will need the 10 gallon filled with water.
Also, watch out for the filter. They do not like strong currents. If you follow my advice and dedicate all the volume to water, you may think about not using a filter.