PS: how do I get one hashtable of two results/queries?

Just add them. But make sure the properties are the same, otherwise you'll get incomplete dumps with export-csv and format-* aso.

$Result = $CasMailbox + $Mailbox

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I believe that autor is asking for hashtable like this

$result=@{
"CasMailbox" = Get-CASMailbox $username | select abc
"Mailbox" =  Get-mailbox $username | select xyz
}

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or probably more something like this:

$result=@{
"a"=(Get-CASMailbox $username).a
"b"=(Get-CASMailbox $username).b
"c"=(Get-CASMailbox $username).c
"x"=(Get-mailbox $username).x
"y"=(Get-mailbox $username).y
"z"=(Get-mailbox $username).z
}

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or the same, but a bit more effective for processing time

       
$CasMailbox = Get-CASMailbox $username | select abc
$Mailbox =  Get-mailbox $username | select xyz     
$result=@{
"a"=$CASMailbox.a
"b"=$CASMailbox.b
"c"=$CASMailbox.c
"x"=$mailbox.x
"y"=$mailbox.y
"z"=$mailbox.z
}

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Thanks.
Howto extend if multiple results?

$CasMailbox = Get-CASMailbox -resultsize 10
$Mailbox =  Get-mailbox -ResultSize 10

Then export to csv?

J.

Note: the+ does not work.
Bot are of different type:
Microsoft.Exchange.Data.Directory.Management.Mailbox
Microsoft.Exchange.Data.Directory.Management.CASMailbox

If they have a different type and different structure, you need to "normalize" them, i.e. create new objects with common properties. You cannot export mixed object types into CSV (that well), as I already stated. And it really doesn't make sense to do a full export of such objects, with their nested object information and real load of data.

So: What do you really want to achieve? What are the generated data to use for?

What I want to achieve:
*get certain properties of a list of users via Get-CASMailbox $username | select abc
*get other properties of the same list of users via Get-mailbox $username | select xyz

Combine them in an $Result and have this exported to a csv so the csv has the specific properties of all those users.

Let me give you an example:

$CasMailbox = Get-CASMailbox | select PrimarySMTPAddress, SamAccountname
$Mailbox =  Get-mailbox | select UserPrincipalName, prohibitsendquota

$ResultPSCustomObject = [PSCustomObject]@{
      SMTP = $CasMailbox.PrimarySmtpAddress
      SamAccountName = $CasMailbox.Identity
      UPN = $Mailbox.UserPrincipalName
      ProhibitSendQuota = $Mailbox.ProhibitSendQuota
     }
$ResultPSCustomObject | Out-File C:\Result.csv

Everything works BUT the Result.csv does not contain the data which is in $ResultPSCustomObject

Just using Out-File is a bad way to get file output. But it should work, if applied to a single object with simple types. And it doesn't generate a CSV format file.

Did you want to provide the same username for both cmdlets, and then mix/combine some of the properties for a single user, as shown in your question?
If you run the commands as shown later, you do not have any relation between CASMailbox and Mailbox properties - they can be from different users.