There seems to be some debate about that. When you mount a daylight scope behind an Apollo or other thermal scope, you are getting no better optical resolution than the optical resolution of the thermal scope. So if it is a 1x thermal scope, any magnification that you get via your daylight scope is just a magnified view of a little display screen of a 1x image inside the thermal scope. So what you are magnification is not optical. There isn't a 5x Zeus but a comparison between a Zeus 3, 640 and an Apollo 640 (1x) with a 3x daylight scope would show that the better image would be in the Zeus 3.
Think of it this way. Which looks better? A big screen TV or the display on your smart phone with a large magnifying glass held up to it? When you blow up the image of the cell phone, or the display screen in a thermal scope, you start seeing artifacts of the display such as the structure of the screen, and not just the image you are hoping to see.