#declare variables
$subscriptionTenantA = "subscriptionID"
$vNetTenantA = "vNet"
$vnetResourceGroupTenantA = "vNetResourceGroup"
$peerAtoBname = "TenantA-TenantB-Peer01"
$subscriptionTenantB = "subscriptionID"
$vNetTenantB = "vNet"
$vnetResourceGroupTenantB = "vNetResourceGroup"
$peerBtoAname = "TenantB-TenantA-Peer01"

#connect to AzureRM
Connect-AzAccount

#select subscription in Tenant A
Select-AzSubscription -Subscription $subscriptionTenantA

#peer vnet in tenant A to vnet in tenant B
$vNet = Get-AzVirtualNetwork -Name $vNetTenantA -ResourceGroupName $vnetResourceGroupTenantA
Add-AzVirtualNetworkPeering -Name $peerAtoBname -VirtualNetwork $vNet -RemoteVirtualNetworkId "/subscriptions/$subscriptionTenantB/resourceGroups/$vnetResourceGroupTenantB/providers/Microsoft.Network/virtualNetworks/$vNetTenantB"

#select subscription in Tenant B
Select-AzSubscription -Subscription $subscriptionTenantB

#peer vnet in tenant B to vnet in tenant A
$vNet = Get-AzVirtualNetwork -Name $vNetTenantB -ResourceGroupName $vnetResourceGroupTenantB
Add-AzVirtualNetworkPeering -Name $peerBtoAname -VirtualNetwork $vNet -RemoteVirtualNetworkId "/subscriptions/$subscriptionTenantA/resourceGroups/$vnetResourceGroupTenantA/providers/Microsoft.Network/virtualNetworks/$vNetTenantA"