Speaking from personal experience regarding dual battery mods..
Sig213 - theres a dedicated thread for this. only big issue is TCR and needing to use Sigelei's values instead of everyone elses
Cuboid 150W - cheap mod. Only issue with this is the 510 pin, its prone to not spring back if you attach an atty with a long 510 connection
Boxer DNA133 & Boxer SX350J V2 - only con is its price $250 a piece. Love their devices
You have a few DNA200 (highest wattage is 133 for dual setup) options for dual battery such as Hcigar VT133, DX200 with 18650 battery case, Boxer DNA133, Efusion Duo, etc.
You have other upcoming options with small form factor dual battery mods such as the Fuchai 213, Pioneer4you iPV 6.