Well I am just a newbie here but I will take the liberty to reply to your question because after all,thatīs what we are here for...
I wouldnīt use the word concentrate in that sentence...you can use it,of course,itīs not grammatially incorrect only that instead of putting it like
"Which one of these two problems needs to be more concentrated on?"
just phrase it as:
Which one of these two problems needs to be concentrated on THE MOST?"
but as I said,it doesnīt sound "everyday english",instead of the verb "concentrate",use the verbs:
which of these two problems needs to be ACTED ON/CONTEMPLATED/GIVEN OUR FULL ATTENTION FIRST/MUST WE FOCUS ON FIRST,MUST WE PONDER OVER FIRST...and then followed by THE MOST but leave out that FIRST.
Of course,"concentrate" fits perfectly anyway you put it...