Because of the different dimensions, you would need to cut holes and seriously damage the larger cage and make it more likely to rust.
If you can find the living world large 3 in diameter connectors that snap onto a cage and a large tube, you could cut holes in both cages and connect them. As I said, this will damage the cage, and do not do it until you have the connectors and tube because I am not sure if they are sold anymore (I am not talking about the tiny hamster ones)
The small cage might make a carrier if you have nothing else but is too small for any animal - not even big enough for a mouse imo
There are plenty of cage ideas on this forum, look at the Reference Thread and the posts in habitat
If that was my cage, I would make a second level that was slightly lower then the top door, and use one of the tiny levels in each section. ……. with those small doors and small levels with pee rivers and chewable plastic ladders, it is not a great cage, but meets the minimum size requirements (base and height) for a rat cage.
It is a "first home" or starter cage …. which means you will want to get a better cage before long
Here is a picture of a temporary cage I added a full level to (I did not leave a space as I was dividing it in half, but I could have left space for rats to go up and down