Nope - fortunately changing the activation key on your Hyper-V host does not automatically change or update the product keys on your Windows Server 2019 VMs. Each operating system instance, including the physical host and each individual VM, maintains its own independent activation state and product key. Changing the key on the host only affects the host itself.
The "1 Host + 2 VMs" rule refers to licensing compliance under Windows Server Standard edition licensing, not a technical feature that automatically distributes product keys to guest VMs. You must still manually or programmatically activate each VM.
Automatic Virtual Machine Activation is an exception. If your host is running Windows Server Datacenter edition and the VMs are running an eligible Windows Server edition, you can use AVMA to automatically activate guest VMs without consuming multiple MAK activations. However, AVMA requires specific generic AVMA setup keys inside the VMs. It is not available when running Windows Server Standard on the host in a standard MAK activation setup.
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hth
Marcin