hullo.
i use heat tape in my racks and they're set up for belly heat.
don't bend the heat tape in anyway that will create a crease. most peeps will tape the heat tape down (w/ foil tape) to hold it in place as it's snaked through the shelves, but some dont. they keep it loose at the sides and just tape down the floor so they can slide the tubs in and out. u can use 1 thermostat for this as long as it's 1 strip and your thermostat can handle the total wattage of heat tape being used.
that's called parallel wiring or daisy chain wiring. i did this w/ the first rack i built myself. make sure each strip of heat tape is the same size and length for uniform wattage and don't cross your wires. this vid shows u how to do it using the clips: https://www.youtube.com/watch?v=y24R0B1n5Sg
alternately, since u are only building a simple 2 level rack, u can use 2 pre-made heat pads if u want. u can run them off separate thermostats or plug both heatpads into 1 powerstrip and plug that powerstrip into 1 thermostat as long as the heat pads are the same wattage.
your welcome. maybe others will chime in as well w/ their tips and tricks.