Hi there. Yes it is a nightmare when you get sold a cage that is too small. We started off with one that was too small as well, and had to upgrade within 3 weeks as our Hamster grew quite large and got stuck in some tubes. On top of that he just seemed depressed. Unfortunately pet shops do tend to sell small cages and are out of touch with pet welfare generally.
The tricky thing now is - buying a second cage to add on isn't the answer I'm afraid. Hamsters need one large continuous floor space - RSPCA minimum recommended size is about 80cm by 50cm. It's fine to add another cage on as long as one of the cages has this minimum floor space. Also most wheels that come with hamster cages are far too small - about 6" and syrians need an 11" to 12" wheel - so the cage also needs to be big enough to fit this in.
You might be able to get a refund for both cages - how long since you got the second one? A couple of people on here have taken them back and got their money back before now.
I've added a photo of our Syrian's cage - which is the minimum 80 by 50 and I have regretted not getting a 100cm cage now! Everything only just fits. He does have a very large house/nesting box in there, and also he has the biggest 12" wheel (because he's huge lol!) whereas an 11" wheel is slimmer and takes up less space.
Zooplus is a good place to get hamster cages. They do one that is within the minimum recommended size for about £30 (the Alaska). They also do the 100cm Barney for less than £50, and that's a great cage.
I do feel for you - I found it physically painful to bin a £40 cage and then spend money on a new one! Ours was a rotostak and I didn't even want to sell it to anyone when I realised how unsuitable it was! But do try and get your money back. At least for the second cage you just bought.
The reason they need the continuous floor space is to be able to have normal behaviours, move around freely and forage and dig. If there are tubes between two cages they can develop abnormal behaviours, block up tubes or even try and nest in them.
Anyway, this is the Alaska cage
Alaska Hamster Cage | Free P&P on orders £29+ at zooplus!
This is the Barney cage
Barney Pet Cage | Great deals at zooplus!
This is the Hamster Heaven, which is virtually the same cage as ours but ours doesn't have all the extras (I did have one of the shelves but took it out). It works best without the tubes, which can be blocked off. It has a good big front opening door, like the Alaska.
Hamster Cages: great selection at zooplus: Hamster Heaven 80 Cage
This is our Syrian's cage and set up, when he had an 11" wheel