You only really need to have 8 insulating joiners if you have a block section after the crossing that you wish to monitor with some sort of feedback system. You will then need to feed power to the outer rails of the crossing too. Otherwise use insulated joiners only on the frogs as the outer rails never change polarity.
You could use the same reversing unit if you can guarantee that you will not have a loco on one crossing and another loco on the other crossing. If there is a chance of the two crossing being used at the same time, use two reversing units.
BTW, I have found in the past that Peco insulfrog crossings work well on DCC, obviously without the need for any reversing unit. They don't foul up little 0-4-0 locos as much as insulfrog points do as with a crossing you tend to be going faster than at a set of insulfrog points.
You could use the same reversing unit if you can guarantee that you will not have a loco on one crossing and another loco on the other crossing. If there is a chance of the two crossing being used at the same time, use two reversing units.
BTW, I have found in the past that Peco insulfrog crossings work well on DCC, obviously without the need for any reversing unit. They don't foul up little 0-4-0 locos as much as insulfrog points do as with a crossing you tend to be going faster than at a set of insulfrog points.