even in a larger tank like a 125 you'll still want to have a divider because the fish will already be mature and will be much harder to pair up then if you'd have grown them up together. I'd personally try to get a 6' long tank and divide it in half or give the male just a bit more room since he'll grow larger.
29....woaaaaaa.way to small for a rd/midas.you should put them in at least a 100gal.good luck with breeding,but get a bigger tank first.......might be easy,might be a nightmare,you can only try,might be supprised.