Two different species housed together is never a good thing. There will be something different somewhere in husbnadry that will negatively affect the other.
Not only that, but how do you think they would react to seeing each other or knowing something else is there?
Let's also consider health. There are certain things found on the skin of certain species that do know harm to that species. BUT, it could hurt another seperate species.
Same with parasites or other microscopic organisms. What if they are fine on one gecko, but can cause harm to the other.
In general, regardless of how similair care may be, two sperate species should never be housed together.