Logically, you are right.
If you've two images with differing dimensions (and differing aspect ratios) from one another, and neither of which have the same length and width (their aspect ratio is not 1:1) there is no logical geometric process by which they can be conformed to a single shared dimension - that predicates that the aspect ratio would be 1:1.
However, I'm sure from @Raphael's reluctance to simply say this, that they know, as do I, that what you can get away with depends entirely upon the content of the images and what you consider the crucial information area of each, and what exactly you mean by "not cutting in" to imagine a scenario in which it might be possible owing to cropping the overall images in such a way (but without cropping any crucial information areas) that they end up with the same aspect ratio as one another... and then you could scale them to be alike.
Hope this helps.