Seems like their stated purpose and even expected times they are operating are different enough that you could have both. An attic fan kicks on during the hot of day and keeps the attic from getting super hot. Then later in the evening or morning hours when it starts to cool down, the whole house fan kicks in to cool off inside of the house. Both result in the attic keeping cooler, which helps keep the house cooler.

The attic fan creates negative pressure in the attic and pulls outside air into the attic through the vents thereby cooling the attic. A whole house fan creates negative pressure in the house, cooling the house by pulling air in from outside through the windows and forcing air out of the attic through the vents. If they were on at the same time, it just seems like the whole house fan air would get blown directly out of the attic by the attic fan, so the soffit vents would just equalize any difference. Should be fine.

Seems like a potential risk of having both an attic fan and whole house fan would be the ceiling vent from the whole house fan would be very hard to seal when the whole house fan is off, so you would risk having the attic fan pull air-conditioned air from the house into the attic instead of outside air which would make your AC work harder during the day.