Nope not quite. It seems to me that there IS something seriously wrong here. The Jagers which are for a 600-1000 ltr tank are 300 watt jobs. Very long and extremely well built. Are these the older Jager ones, or the newer 'eheim-jager' ones?
Regardless of older model or newer one, only one heater is required for that volume of water (Assuming the tank is inside a house, room temp above ~18%), the second heater is there as a backup only.
If both these heaters are 'always on' and still not heating your tank to 28C then the heaters are at fault. Time to get tough with your local LFS - or whoever sold you the heaters.
Finally, you have depressed the 'calibrating' button back in havent you? At the top of the heater neaest the cord...this button must be pressd in for them to work correctly.