Well, so what nature are these 200 by 300 inputs? Are they independent or is there some notion of locality in either dimension such that a thing and the thing next to it have more relation than the thing and the thing 100 places further? Do you expect your prediction to depend on the relation between them?
The things with locality should probably be treated as kernel dimensions similar to width and height of an image. Things without connection could either be more of “linear layer nature” (i.e. you consider all of them at once) or consider them indivdually (so you would have a kernel size of 1 in that dimension).
If you have locality in both directions, you could just use unsqueeze on the input to make it a single input channel.
Best regards
Thomas