Welcome HappyBetta to the world of fish keeping and to the forums.
Please take to heart the advice people here have given you so far.
But I will give you my thoughts and opinions as well to give you more food for thought.
That tank is half a gallon, not entirely a good idea for one betta, let alone two.
I am not one who will argue too much on what size of tank is needed, as there is no true minimum of tank size- but matters of how they are cared for.
For the tank you are looking at here is what you will expect:
With two bettas in it, you will want to be doing every other day water changes- and when you do them, you will have to cup each one and change out all of the water, then you will have to somehow float and acclimate them both to the new tank and chemistry which would be very nearly impossible in that size.
The frequent water changes will stress out the bettas.
Being so close to another betta in a confined place with no where to hide, and no room to actually SWIM (which fish do and need to do........) you are putting them through a lot of stress.
A stressed betta will turn into a sickly betta.
A stressed betta will not dance for you, or do tricks such as eat out of your hands.
A stressed betta will not be as colorful.
A stressed betta can easily develop illnesses for which you will have to buy another tank to care for them.
A stressed betta will not live as long.
That tank you will not be able to heat correctly- bettas are tropical fish coming from BIG expanses of water in a hot climate.
The tank set up you saw at the store was only a temporary set up- most likely they have to replace one every couple of days due to selling one, or one dying.
These tanks make great hospital tanks (when divider is removed), but not permanent homes for two bettas.
With two bettas in a small container the ammonia and nitrates will build up quickly- which will cause death.
Bettas are sold in cups because it is the best way to keep them safe- and it's not feasible to give a betta a whole display tank to just one. But the cups are not an indicator on how they should live.
Think of this: You see goldfish bowls around.. but a goldfish grows to be over a foot long, needs very high filtration and can live 20+ years.. so would it be smart to give it a small bowl that it cannot grow and swim? Goldfish in those bowls do not even live a quarter of their life span- but there are goldfish bowls. Why? Because people will believe anything if they have not done their research and will buy those bowls. All it does is kill them since it does not give them what is necessary to keep them alive and healthy.
Bettas are living creatures, just the same as cats and dogs and birds, etc.. they are just a different species. They recognize their owners, they interact and play with their owners and they show their owners affection. The only difference is you can't house train them or play fetch. Caring for fish, just like any animal, you must do what is needed in their basic care.
Bettas do great and just fine in smaller bowls, 1 gallon and up is what you are wanting- to be able to heat and to give them the mental stimulation such as plants and a cave, etc. (Not to mention the spiky plant that comes with that set up will tear and rip the fins big time.)
It is very much recommended you do some more research before purchasing two bettas and placing them in there. Don't believe the people who just want to make a sale- listen to the people who care and who take care of these little ones.
I recommend heading to PetCo, buying a medium to large kritter keeper for $10 and under, and a heater. If you want to have a pet, a living animal, then you must be responsible for it and give it the proper care and environment they need to live healthy and a full life. These aren't decorations. The only difference between a betta and any of the other tropical fish you see swimming in the bigger tanks is that these guys are fine in a 1+ gallon.. other then that, they need warmth, room to swim, weekly water changes, places to hide, etc.
Next time you're there.. ask the employee how often they have to scoop out a dead betta from that "display" if they only change half a gallon for two bettas only once a month. You have to realize they don't tell these employees anything about taking care of the animals when they hire them- it's up to the employee to do their own research and learn.
You have people on this forum who have been raising and breeding these fish for decades. So.. I recommend listening to people who know rather then someone who gets paid to sell living creatures without knowing anything about them ;)
Also, please don't think you need 2.5 gallons.. that is in no way the minimum, just a preference. I keep all mine in their own 1-2 gallons just fine and healthy- but I also do 2x weekly water changes and they are heated. It's easier to heat larger tanks.. but all in all, it's about the care you give them. It's just not ideal, nor could you give two the care needed in such a small container.
I'm not trying to harp, or to tell you what you have to do- just trying to give you the basics on what is needed to keep these guys alive and healthy and hope you make some good choices in doing so.