The two stools bear absolutely no relation to one another. Pick one.

IMO, those aren't cohesive. I wonder if your back & backless idea would work if they were the same brand? I can't find a picture of mixed stools though, so maybe there's a reason.

NO.

No, do not do that. I would only put 3 bar stools at the island, all the same, and they need to have a back on them. No one wants to sit on a stool, it's not comfortable above the age of 6.

Since stools are such a leggy piece of furniture, I think the legs should match in terms of stain/color. I agree with camilleclaudel that if it's not an exact match it looks like you tried to match but weren't quite successful. The upholstered chairs with the back are nice. The wooden stools, however, don't quite match or coordinate with them. If you must have a pair of stools, I'd personally go with an upholstered pair in a fabric that coordinates with the 3 main chairs, perhaps a patterned or boldly colored fabric.

Of course you can "do whatever you want". The op asked what would look good. The two stools do not look good together as they are vastly different in style and feel. .....sharing nothing but the fact they are indeed.......stools.

@Jan
I understand that's your opinion. My opinion is, without seeing the kitchen, I can't tell if they look good or not. They don't look bad to me in the mock-up. You can't tell by seeing them in a vacuum. The three might tie in with one element in the kitchen, while the two tie in with another. Obviously, taste is subjective. I don't object to mix-and-match if it works. I dislike all matchy-matchy when it feels sterile or pedestrian. I don't object to personality thrown into a space. I don't think they look bad. I, personally, hate stools of all kinds. I find them extremely uncomfortable and pointless. But, I understand it's some people's preference. All I'm saying is taste is personal.

@Adil
Really, the only way to know for sure if you like them is to get them home and see them in the kitchen. You'll know right away if they feel wrong or right to you.
I disagree that the stools have nothing to do with each other. Although the curves are different, the legs are similar, not the same... and that's a cohesive factor...if they're the same wood and stain/color. I can't tell from the pictures.

I wonder who the poor person is who gets the backless, hard stool.

Thanks for all the feedback. Wow, I didn't know backless stools were hated on!

Also, people mix and match dining table chairs with totally different fabrics/styles. So I thought maybe it would be okay for bar stools on kitchen island too.

Backless stools are not "hated on" There are a zillion backless stools. There are LOW back stools that meet the need for an uncluttered feel as well. ....and still provide support, while the back remains below the counter top. Google is your friend, we can't read minds, nor have you shown us your kitchen and island. You're just showing us two UNRELATED stools. A better choice is one style that functions all sides. None of these are taller than 35 inches

Chi Counter Stool, 25.75" , Brushed Stainless Steel Frame, Gray · More Info

I'm looking for these bar stools. I need 2 have 2 more. or else I have to mix with my backless leather or find something else to do with them.
Stephanie

Use Google Lens. It’s magic and can find ANYTHING for you!