I can't say you won't have problems, but if you introduce them slowly
and let them work out their relationship without showing favoritism,
things should be just fine.  Be sure to play with the playful cat as
much as you pet and cuddle with the affectionate cat.

I once had a very large and mean (neutered) male cat.  I brought home a
5 week old kitten who was the runt of two litters where one mama was
nursing 15 kittens.  He wasn't getting much from her.  Add to that
every parasite in the book and conjunctivitis... I figured he was
either going to die from all that or my other cat would kill him, but
he was going to die where he was so I figured I'd try.

The kitten, Calvin (who lived to be 20!!) took one look at Lamonte and
made his little mama mew and ran to him.  Lamonte was at least 4 times
as big as the mama, but both were black.  When Lamonte got over the
initial shock, he actually began to mother Calvin.  He let him suck on
his teats, he cleaned him to stimulate defecation, and he bathed him.
Calvin lived a very long time very happily with his new "mama".
Lamonte was very active and playful and aggressive, and Calvin was very
sweet and affectionate. They had daily "rage out" sessions all their
lives, which shocked guests with their fierceness, and they cuddled
each other, too.  My current 3 year old doesn't mother the new kitten
quite to the same extent, but they do groom each other as well as rage
out together.  Hopefully you'll end up with two brothers who at the
very least tolerate each other.  They'll probably get along fabulously.