What do the chords in parentheses mean, and how could you play Bm7 and B♭M7 at the same time?
Several answers point out that this is an alternative progression. An important point is to question why. You definitely do NOT play them at the same time. When this type if choice is given you may have one of them during the solo section to provide more "harmonic texture" if that's a term, or they may be for different endings. It would help to see the next line to know where the chromatic ii-V's are going. Is the tune modulating to Ab? This type of device is very common in Wes tunes as the head may have a simpler set of changes and the solo filled up (chromatic ii-V's are common in Jazz and Bop). The other common use is for the ending of the song (i.e. "on the final time through the head").
They are just alternate chords NOT TO BE PLAYED SIMULTANEOUSLY. It is a ii V7 chromatic progression which is very common in jazz.
It is a "longer" way to get to the I chord. So instead of playing a boring Dm7 to a G7 to a C, you can play, Ebm7 Ab7 Dm7 G7 C. Or Em7 A7 Dm7 G7 C. Or . . . there are dozens of variations. You can do whatever you like. That is what will make you you.
Listen to players like Oscar Peterson, Art Tatum, Dick Hyman, Peter Nero and Adam Makowicz, they are all prolific ii V players. So instead of playing the written ii V they will go up a half or whole step, a third or a fourth, and work down to the I.
All music is littered with two fives. It is the base of the circle of fourths. As far as standards go, SATIN DOLL or LOVER are loaded with them.
As an exercise, pick a V7 chord and use four chords to two five down to it.