Hi!
The Double Voltage Hub only works with 2 DELTA Pros, and it won't work with 1 DELTA Pro + 1 DELTA Pro Smart Extra Battery because the extra battery doesn't have an invertor, it just increases the capacity of DELTA Pro.
EcoFlow Double Voltage Hub (DELTA Pro only)
Double your capacity: Plans for using tons of energy off-grid? Link two DELTA Pro units and their battery management systems will distribute capacity evenly to anything plugged into this hub.
Even more output: House battery backup never had this much output. Chain two units together and your DELTA Pro’s will output a whopping 7200W, enough for even the most heavy-duty of appliances.
Portable & lightweight: The double voltage hub is easy to carry, and pairs with your DELTA Pro to bring even more versatility to your portable battery set up. Charge up, link your power stations and you’re good to go.
Three more outlets: The hub has two NEMA 6-20 outlets that power regular electronics as well as 20-amp power tools. It also has a NEMA L14-30 outlet, making it that much easier to switch out your old gas generator.
Note: The Double Voltage Hub requires two EcoFlow DELTA Pro units.
What's in the box

1. Double Voltage Hub (Not to scale) 2. User manual
Specs
Input Voltage
100–120V∿ 50Hz/60Hz
Output Voltage
200–240V∿ 50Hz/60Hz
Max AC Input/Output
US version: 7200W, 30A