It'll depend on your distance to target. If you set the two crosshairs to agree at 20yards, they'll be slightly off at 10 and 30 yards. Because the thermal is further from the bore, it would be pointing to a target lower than the red dot at 30 yards, and above the red dot at 10 yards.
View attachment 566008 Here is a rather sketchy illustration. The projectile travels in a roughly parabolic trajectory. The red dot shows you light, which travels on a straight line. So there's three points of intersection (*). The main thing is before the cyan and red lines intersect, the cyan is above the red. Afterwards, the cyan is below the red.
(*) Technically if I drew this figure out far enough there'd be an additional two points of intersection where the projectilie comes back down and crashes through the colored lines again.
This additional information of the two lines not always meeting up can be used as a tool. If you're on-target and the thermal is a bit above the red dot, that means your target is closer than the distance where they meet. So if you've measured that distance ahead of time, it could help you compensate for the distance, i.e. it's a primitive range-finder. Similarly if the red is above the cyan, you know the target is beyond the distance where they meet.
I sometimes use this strategy in other ways. On a short-range pistol I have two tiny laser pointers. One is green, one is blue, and they're mounted on the sides of the pistol. So if I'm on target with the two pointers and green is to the right of blue, I know my target is closer than 20 yards. Reversed, and I know my target is beyond 20 yards. They're zeroed for 20 yards, so with this pistol I can often just shoot from the hip.
