Give Fred 3 or 5 bullet holes - problem solved.
OK, seriously though :
1) Actively engage with Fred, and the rest of the band, to listen to the effect of both instruments playing in the same range, and decide if it's a help or a hindrance (we already know the answer, but they need to have that epiphany themselves).
2) Figure out when both playing in unison adds power, and excitement (it can work really well, but both players have to nail identical parts - the keys need to be using either a punchy staccato synth sound [think typical Moog type sound, with adjustable resonant filter, with a short attack & release envelope] or something with rich harmonics [such as a bright piano] that takes overdrive / distortion well).
3) Figure out passages when one of you can hold down the low end, and the other can venture higher up (punchy synth bass sounds great with a bass guitar playing high melodies [Joy Division / New Order made a career of this], or conventional bass parts combined with a synth pad filling out the mid-range can give the guitars ample opportunity to stretch their legs, before the synth lets rip another octave up, with a lead sound).
Record your rehearsals, and highlight the moments that work, and those that don't (keep pointing back to this evidence, if things start to slip).