Going with what VaporDNA says about the Topbox mini kit, comes with 3 coils
1. 0.15 ni200
2. 0.5 ssocc
3. 0.5 clapton
Regarding your currently installed 0.15 ohm coils. It is for TEMPERATURE CONTROL ONLY. I would suggest not to vape it under wattage mode like you currently are doing.
Regarding your question as to why they've included 2 different 0.5ohm coils. Not sure which SSOCC 0.5 they come with but it could be stainless steel, with stainless steel you can vape under wattage or temperature mode. With the 0.5 clapton (assuming kanthal material) you can only do wattage mode.
Clapton coils are basically 2 wires with 1 being the base and the other 1 wrapped around it. It provides a larger diameter wire thus increasing the heating surface of the coil, which some would argue produces better flavor and more vapor as compared to a single strand (but in turn requires more power to heat up the coil).