Start by running more subscribers for one subscription:
dotnet run --project src/servicebus/subscriber -topic broadcast -subscription desktop -cs '<subscriber-connection-string>'
The new subscriber doesn’t start with a backlog of messages to process, because the other consumer has been working through the subscription.
New message batches are shared by the subscribers:
web subscriber processes all the messages when they hit that subscriptiondesktop subscribers share the incoming messages, just like with multiple subscribers on a queueAdd another publisher:
dotnet run --project src/servicebus/publisher -topic broadcast -cs '<publisher-connection-string>'
Both subscriptions get copies of all the messages from both publishers. The processing continues in the same way, with incoming messages shared between all the consumers for a subscription.