First, I'm not really a cactus kind of guy, so if I say something stupid, please correct me... Those both look like they are of the same species. I would guess Mammillaria spinosissima since it is both very common in cultivation, and basically a visual match. It really is just a guess though. I suspect those two plants came from seed and it was just easier for the grower to keep them together than to tease them apart to be sold individually. I suppose one could be an offset of the other, but they seem much too young/small to be offsetting. I personally wouldn't bother separating them, but if you are careful, I don't think it would be a problem. Keep an eye on the smaller of the two as that yellowing area is a bit worrisome. If it turns out to be rot, get rid of that plant quickly to prevent it from somehow spreading to the other. As for conditions, they want little water and lots of sun. My understanding is that blooming cacti indoors can be pretty challenging. Many species need hot days and cold nights to set blooms, conditions that are difficult to achieve indoors. Hopefully someone with a bit more experience can shed a little more light on this subject.

ad sorry forgot toadd the link. mine is like this but not as tall Here is a link that might be useful:

Spike, on photobucket, you have to use the html code to get your pictures to show up.

When I lived in Los Angeles, the Mammillarias loved life! Mammillaria theresae Mammillaria supertexta Mammillaria bocasana Mammillaria camptotricha Mammillaria carmenae (this was actually a tough plant to keep alive even in Los Angeles and eventually it rotted… needs a greenhouse I think) Mammillaria elongata monstrose (nice plant but never flowers) Mammillaria mystax MUCH happier back in Los Angeles (same plant as above) Mammillaria elongata cristate form (also a hard plant to keep alive for many years in a row) Mammillaria formosa… spines so small no gloves needed to handle this one Mammillaria gracilis is an easy plant, but falls apart easily, too