Hello and welcome
You are not failing at all, you have unfortunately just not been very well advised by the pet store you got your lovely hams from.
You have two completely different species of hamster there, your first hamster is an Agouti Robo, and is a boy, as he is still young he doesn't have his scent gland yet, and his testicles aren't visible, they will develop within the next few weeks.
Your second hamster is a Russian hamster and is a girl. Robos never have a dorsal stripe, and as you have noticed she is very both shape and size wise very different. This difference will become more apparent as they grow as well.
It is important they are not housed together, hamsters of mixed species must never be housed together, it is dangerous for both hamsters. In my opinion you have a case with the information you have to ask for a discount or even a free cage for your other hamster as the pet shop really should know their species.
If not, there is a bargain cage thread on the forum with lots of cheap cages some of which will hopefully be near you, there is also the option of the bin cage which members here can help you with.
Again I must stress that this is unfortunate and not your fault, but it is imperative that these hamsters are separated.