When it comes to series mods you have to QUADRUPLE the resistance to get the same wattage vape.

If you use ohms law with 4.2 Volts and 0.20 Ohms resistance you will theoretically pull 21 Amps from your batteries. Multiply 21 Amps x 4.2V will give a theoretical 88 Watts. Of course battery sag and voltage drop will take their toll and in reality you'll get something in the mid to high 70s range when the batteries are fully charged.

If you do the same with 8.4V and 0.4 ohms you will still be pulling 21 Amps, but when you multiply the Amps by the Voltage to get a Wattage figure you multiply by 8.4 instead of 4.2 - this gives a theoretical 176 watts, even with battery sag and voltage drop that still is going to be around 160.

Personally I would forget pre-made series coils as they tend to cater for high wattage only, you could use dual 26 gauge single claptons, but you will have to use a lot of wraps to get anywhere near a 0.80 build - and they may be a bit much for an enclosed RTA deck.

Personally I would stick to dual coil RDAs with plenty of airflow and the ability to take wide coils, but don't forget to QUADRUPLE the resistance for the same wattage vape, if you feel you need more power it is easy to knock off a wrap or two - there will still be plenty of coil to cotton contact.

I suggest you try dual 10 wrap 3.5mm i.d. claptons in an RDA and work it out if you need more or less wattage from there.