Can I use multiple MOSFETs with one 555 timer?
No, the 2SK1941 would be a poor choice due to it's high on-resistance. If you paralleled two devices and assumed that both would take 6.5 amps each then the forward conduction characteristic would be like this: -

I added the red lines.
With 6.5 amps through one device, the volt drop would be about 2.2 volts and the power dissipation would be 6.5 x 2.2 = 14.3 watts. Using two devices means a total power dissipation of over 28 watts.
That's a lot of heat to get rid of and 28 watts represents nearly an 18% waste of heat and that's pretty poor for this type of circuit. I would certainly choose a better MOSFET and you'll probably only need one device.
You should be able to locate one with an on-resistance around 5 milli ohm and that means a power loss of 13² x 0.005 = 0.9 watts.

Is it going to be very complicated to split the load evenly between
the MOSFETs

That's always something to consider and can be problematic hence why I suggest one MOSFET and much higher efficiency.