More a matter of astronomical observation (pun intended) than anything else.
In binary star systems where two stars are that close together, one would orbit the other - thus one would always be smaller than the other except on the occasion that the planet passes between both - in that case you would only see one (or the other) since they would be on either side of the horizon, one east one west.
Most like 99.9% of the known binary star systems are composed of a large (red giant for instance) star and a small (white dwarf) star. Two equally sized stars rarely reach a point of stability to where they maintain their sizes/colors even a relative apparent size in relation to a planet bound observer within that system. One star would, in a close pair, 'suck up' or eat the other.
Thus either by a very real size difference between the two stars, or simply because to the observer on the planet one star is further than the other one of those suns needs to be smaller (perhaps even tinting a light blue, or other color to suggest different type stars).
Second observation of perspective. the lowest sun really looks to be in front of the background - ergo not really a star but some other light source.
I get the feel that the distant landscape is hills, yet the sun(s) are somehow in front of that landscape. Since you are using hills on either side, it is expected that there are hills where those 'suns' are.
Actually both the left and the right one appear to be in front of their hills. The left appears to be sitting on the hills, not rising or setting behind the hill.
Contrast and shadow. Non-existent in your image. By all rights we should only see black or near black trees directly in front of the sun(s). the overwhelming light should make color and texture of trees near impossible to see. So again we get the impression of light sources in the foreground, not rising/setting stars/suns.
Thus 'issues' with hills may not actually be that the hills themselves are bad, but that the suns do not 'feel' right in their placement against/behind said hills.