It's possible that you will have a problem with your car's wiring not supporting enough amps for all of those machines running concurrently. 2 laptops and a mini running all at the same time could be over the typical 10 or 15 amps (== 120-180 Watts at 12 Volts, minus the inefficiency of inversion from DC to AC and some other factors) that DC outlet circuits in most car electrical systems are rated for.
The gauge (thickness) of the wiring used determines the maximum load it can safely convey; based on this, manufacturers provide fuses or circuit breakers that ensure that the load will not exceed that level. Putting too much current through a wire will cause it to heat up, like a light bulb filament or a stove burner, to the point of being a fire hazard.
Some cars have DC outlets on more than one circuit; if this is the case with the Innova, you may be able to avoid such problems by hooking the different machines up to multiple DC-AC inverters plugged into separate circuits.
Otherwise, you may need to connect your second battery up to a new circuit and make sure that you use wires that can easily take the load you're planning to put on it (and use fuses or breakers to avoid fire hazards in the case of overload).