I have been working very closely with BitTitan for a number of years and BitTitan have been working very closely with Microsoft with the development of their Teams migration service.

The service was updated towards the end of August and this update brought a large number of enhancements which can be reviewed HERE

For years I have been migrating Office365 Tenants to other Office365 tenants and the problem still remains whereby a custom domain like contoso.com cannot exist in two Office365 tenants at once. So during acquisition or merger migration projects at what point do you migrate the Microsoft Teams sites. My recommendation is to take care of the following data sources first with the Migration Wiz user migration bundle.

Primary Mailbox
Archive Mailbox
OneDrive for Business
Outlook switch over via deployment pro.

Once the data sources above have been migrated , I would recommend that Outlook Web App access to the legacy source tenant mailboxes is blocked via running the following command

Get-Mailbox -resultsize unlimited | Set-CASMailbox -OWAEnabled $false
Get-Mailbox -resultsize unlimited | Set-CasMailbox -ActiveSyncEnabled $False

The official MigrationWiz migration guide is available HERE
Also follow this ARTICLE and setup the  Teams-FullControlApp in each source tenant.
I recommend that you use the autodiscover method to populate the project as this will also identify any incompatible items in Teams sites or channels

First step is to create the teams site in the target tenant , and do this 24 hours in advance

Next Step is to do the data migration

Validate the data and then remove the Teams licenses from the users in the source tenants with the following powershell commands. Create a new variable for each Offfice365 licensing sku that contains Teams.

Get-MsolAccountSku

$acctSKU=”contoso:O365_BUSINESS_PREMIUM”
$x = New-MsolLicenseOptions -AccountSkuId $acctSKU -DisabledPlans “TEAMS1”

$acctSKU2=”contoso:TEAMS_EXPLORATORY”
$x = New-MsolLicenseOptions -AccountSkuId $acctSKU -DisabledPlans “TEAMS1”

$acctSKU3=”contoso:O365_BUSINESS_ESSETIALS”
$x = New-MsolLicenseOptions -AccountSkuId $acctSKU -DisabledPlans “TEAMS1”

$acctSKU4=”contoso:O365_BUSINESS_ESSENTIALS”
$x = New-MsolLicenseOptions -AccountSkuId $acctSKU -DisabledPlans “TEAMS1”

Get-MsolUser | Where-Object {$_.licenses[0].AccountSku.SkuPartNumber -eq ($acctSKU).Substring($acctSKU.IndexOf(“:”)+1, $acctSKU.Length-$acctSKU.IndexOf(“:”)-1) -and $_.IsLicensed -eq $True} | Set-MsolUserLicense -LicenseOptions $x

And now all the Teams sites have been migrated and there is no chance of split brain because the Teams license has been removed from the source tenanat.

One really important point to note: MigrationWiz match users from the source to the destination based on the user prefix which is so useful when you are moving one domain from an Office365 tenant to another for example in a merger or aquistion.