Step 1: Remove peering between Vnet1 and VNet2.
You can't add address ranges to, or delete address ranges from a virtual network's address space once a virtual network is peered with another virtual network.
To add or remove address ranges, delete the peering, add or remove the address ranges, then re-create the peering.
Step 2: Add the 10.44.0.0/16 address space to VNet1.
Step 3: Recreate peering between VNet1 and VNet2
Reference:
https://docs.microsoft.com/en-us/azure/virtual-network/virtual-network-manage-peering