If both earths/ grounds are bolted to a known good chassis connection, then there will not be an issue.
Problems arise if two different wire gauges are simply spliced to an existing earth/ ground rather than bolted to the same point as voltage drop and feedback issues can occur. This is due to higher current draw on one of the circuits.
Many fibreglass vehicles and insulated return vehicles have earth/ ground wires running to a number of locations.
The aviation standard is no more than 3 wires to a ground stud. So 2 should be just fine.
Simply make sure it's clean and tight. I like to spray a bit of corrosion inhibiting compound over the assembled ground points if they are exposed to the elements.
It"s often done, and recommended, in the communications world. It's known as a "single point ground". It ensures all the equipment is at the same electrical potential.
All grounds should be on the same voltage, no interaction possible. However the worse the ground is the higher the voltage becomes at the ground connection