It's likely showing both popups in the same place on top of one another - if you look at the default achievement_popup screen, there's a special bit:
## Allows multiple achievements to be slightly offset from each other. ## This number should be at least as tall as one achievement. default achievement_yoffset = num*170 frame: style_prefix 'achieve_popup' ## The transform that makes it pop out at achievement_popout() ## Offsets the achievement down if there are multiple yoffset achievement_yoffset
That yoffset bit makes sure that if there are multiple achievements shown at the same time, they get offset so they're not stacking on top of each other. Make sure you have that code, or something similar, in your own popup screen!