A contactor switches 800 amps from two 26 volt batteries in series in a resistive circuit. Can I use a mosfet
If you want to have 800 A driven by 26 V, then that implies that your load, including whatever you use as switch, has only 32 mΩ resistance.
That will most definitely be a challenge to do with a MOSFET as such a low voltage.
I doubt this is physically possible, however, for a different reason: your batteries source impedance needs to be significantly lower than that. And such batteries do, to my knowledge, not exist.