Re: Combining two 8 AWG from battery to inverter
Don't do this. It is a bad and unsafe practice to parallel inverter feed. If you parallel wires each must be fused according to its maximum current rating. Otherwise one may increase in resistance causing the load to shift onto the other - resulting in an overload of an individual wire before the whole circuit is overloaded to a point where the circuit protection trips.
One 8 AWG wire can handle about 50 Amps. Two would handle 100 Amps.
Wiring for an inverter should be based on its maximum output power divided by its minimum input Voltage. In this case 2500 / 10.5 = 238 Amps - more than double what two 8 AWG wires would handle.
Do it right: get the bigger wire. You need at least 3/0 (000) and preferably 4/0 (0000). Five feet of that won't cost so much as to make it untenable. It would certainly be cheaper than starting a fire by overloaded a cobbed-up bunch of paralleled 8 AWG.
Four 175 Watt panels, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.
Ohm's Law: Amps = Volts / Ohms
Power Formula: Watts = Volts * Amps