Aiiiii! Not sure that's a good idea at all. Pulling air from the top could interfere with exhausting air from the inside as well as draw in higher warmer air in the room, and theoretically pulled in from the exhaust fan's output in the back. The Plus already has two vents on the chamfers. Traditionally, fans were installed in the Plus on the chamfer side opposite the analogue board. This has the effect of pulling air from the warmer side to the cooler as well as exhausting it as far away from the warmest side as possible. Having two fans will likely only increase noise and air turbulence. It is almost a guarantee that they will spin at slightly different speeds possibly setting up annoying frequency oscillations. I admit all of this could be negligible impact, especially if all you really want is twin exhaust ports, like a dual mag exhaust on a '67 Chevy Impala.
Frankly, I would stick the whole thing in the Plus, cut out the port panel, and otherwise do as little modification to the case as necessary. If Apple had glued the port panel on in the first place the way the top vents are, the upgrade from the 128K to the Plus would have been a lot less wasteful.