Adding another inverter
In short
A home can have several inverters. Add each one separately, then tell Svitgrid if they share a battery.
Settings → Add inverter, then the same steps as the first one: model, connection, battery, panels.
Because the household already exists, you are not asked about location or collector again.
Parallel systems
If your inverters are wired in parallel, you will also be asked about topology — which is the master and which are slaves.
Shared battery banks
If two inverters are connected to one physical battery, say so in each inverter's settings. Otherwise Svitgrid counts the same battery twice and reports double your real capacity.
How many are allowed
Your plan sets a limit — see Plans and subscription.