Paralleling mosfets to handle more current while driving a DC motor?
Yes, you can parallel MOSFETs, they are good at current sharing due to the positive temperature coefficient of their on-resistance, but... 

You've made an error in the power dissipation estimate. 

In PWM operation, the MOSFET is either fully on or fully off.  The motor acts as a low-pass filter, and "sees" the average applied voltage, the supply voltage times the PWM duty cycle.  So at 24V supply and 40% duty cycle, the motor would behave as if 9.6V dc were applied.  I should mention that the PWM frequency should be high enough to keep the current ripple to a reasonable level.

When the MOSFET is on, the voltage across it is due to its on-resistance, 0.16 ohm at 25 deg C according to the data sheet.  This is called conduction loss, and it varies with the current and the duty cycle.  For example, at 2A 40% duty cycle the power would be 2A squared times 0.16 ohms times 40%, around 250mW. (This could more than double at higher temperatures; in any case, the power will be far less than 28W.) During this time, diode current is zero. If conduction losses are too high, you could select a MOSFET with lower on-resistance.

When the MOSFET is off, the current is flowing through the diode, and the MOSFET is dissipating no power (but the diode is).  

There will be some switching loss as well, in both the MOSFET and the diode. Switching loss is a function of switching speed, which in turn is a function of the gate driver.  Driving the gate harder reduces switching losses in the MOSFET, but can give rise to ringing due to parasitic elements.  For best operation, parasitics must be minimized, so a tight layout is critical.

You will definitely need a diode with a higher current rating than the 1N4148. At 24V, I'd suggest a Schottky type for lower forward voltage drop.