Awaiting the arrival of a fugitive, a ruthless bounty hunter holds hostage the patrons of a sleazy bar but soon discovers the imminent arrival of another fugitive: a killer with advanced mental powers from an alternate reality.

Nice suggestion. My use of 'another' is ambiguous. I meant 'one more' and not 'instead of'.

"A bounty hunter holds bar patrons hostage in order to trap a fugitive but when a killer with superpowers from an alternate reality also shows up the hunter becomes the hunted".

Like that as a logline twist: the hunter becomes the hunted. (Maybe someone wants to collect a bounty on him.)

"A bounty hunter holds a bar of patrons hostage in order to trap a fugitive but when another killer from an alternate dimension shows up instead- the hunter becomes the hunted."

I don't even know if that is true- but I got the feeling that is kinda what happens.
Removed the part about the mental powers because it felt like trying to squeeze too much into one line. I guess you could go with:

"A bounty hunter holds a bar of patrons hostage in order to trap a fugitive but when a super powered killer from an alternate dimension shows up instead- the hunter becomes the hunted."