[Xack]

Have you considered a battery isolator? "Single battery isolator utilizes the energy produced by the alternator and distributes the power evenly to each battery. Single battery isolator allows primary batteries to charge together but isolates each battery during operation." This will maintain your starter battery and allow only the starter to be driven from that battery even though your electrical system is run from perhaps a smaller Lithium battery. I would think there should be a device on the market for connecting batteries in parallel to control overcurrent when they equalize after loading or charging. I would never recommend different sized batteries connected together without some kind of regulator device between them.
Same Kind batteries in parallel should not be a problem as long as you do not have more than a few volts difference between them. Normally, your charging system will have 14 - 15 volts to charge your battery, while your battery will have 12-13 volts at rest. The 2 batteries will equalize over time, one discharging into the other. The main problem will be at starter demand, if the 2 batteries do not discharge at the same rate under load, then you can get current loops even after starting has completed. Potential differences in the cells can have high currents which result in heat and maybe fire. Exploding batteries is not good for your Cobra. I guess the major concern I would have is if you have 2 of the same batteries and one goes bad causing discharge into the bad battery. That would probably be the worst case scenario. And that all depends if your battery shorts...?
Isolators just create 2 or more circuits in your car. Battery 1 for starting, Battery 2 for other items.