Minis vary in size just like hybrid teas, floribundas, and other categories do. I doubt mine is the answer you're looking for, but I wouldn't put two in the same pot unless the pots were enormous and you knew you would never have to separate them again. I have about twenty different minis, and a couple of them are in 18" pots (Miss Flippins and Overnight Scentsation) because they simply grew bigger than others I have. Most of mine are happy in 14 to 16 inch pots. The very smallest I have (Y2K and Fame Parade) are in 12 inch pots.
They don't need that much space at first. But the problem with putting more than one in the same pot is that by the time you need to separate them, the roots will be so intertwined that they're likely to be severely damaged in any attempt to separate them. It may seem like the baby roses look too small for those pots, but they'll actually grow to fill the space faster than if you planted them first in smaller pots. You might consider planting some non-invasive small annual flowers around the baby rose in the pot until it fills out more. I think 8 in. pots are often ok for first-year minis, but they'll start to outgrow it by the end of the first growing season.

I have grocery store minis that are three years old (all in the ground). Some are 12" x 12" and some are 3' x 3'. Unfortunately, there's no way to know which way yours will go because they aren't labelled with their variety names. If you have named minis, you can look up information on them on helpmefind.com/roses

WOW...I meant 12", not 12'! LOL!

Anyhow, thanks for the replies. I've decided to just enlarge my garden and put them in the ground except for the most gorgeous one which will go into a 16" (that's inch! LOL) pot on my patio. Thank y'all for your input. BTW they're almost all grocery store roses, but even the 3 I got at a real nursery don't have names on them.