ebike conversion kits

I've been looking at different e-bike conversion setups lately, and the more I read, the more it seems like choosing a kit based mainly on motor wattage is probably the wrong way to go about it. At first it's easy to look at 500W, 750W, 1000W, etc. and assume that's the main decision. But after...

I have a 36v 500w front wheel ebike now and want add another rear wheel 36v 500w kit and run both kits to one throttle powering both motors to one battery, but I'm confused about the battery. What size battery should I purchase for this type project that will not damage the motors, controllers...