Weigh in before eating. Empty. If not then the weight is unreliable.
If you have to feed 2 rodents in one sitting then its time to move up in size. Medium mice up to large mice. I don't see your little one downing a small rat without trouble unless its really small. Small Frozen rats are pretty small so that might be an option
Mice have less protein than rats. Rats less than African Soft Furred. Though ASF are hard to come by most places and max out at the size of a small rat.