$EP = New-AdfsSamlEndpoint -Binding "POST" -Protocol "SAMLAssertionConsumer" -Uri "https://test.com" -Index 1
$EP1 = Get-ADFSRelyingPartyTrust -Name "X" | Select-Object -ExpandProperty SamlEndpoints
Set-AdfsRelyingPartyTrust -TargetName "PsTest" -SamlEndpoint $EP,$EP1

Get-AdfsRelyingPartyTrust -Name "X" | Select-Object -ExpandProperty SamlEndpoints | Select-Object -ExpandProperty Index

$EP1 = Get-ADFSRelyingPartyTrust -Name "X" | Select-Object -ExpandProperty SamlEndpoints | Where-Object {$_.Index -eq 2}