もしpowershell2ならこれなら動くんじゃないかな、試してないけど
@($a; $b) | Group-Object -Property Name | Where-Object { $_.Count -eq 2 } | Foreach-Object { $_.Name; Compare-Object -ReferenceObject $(Get-Content $_.Group[0].Fullname) -DifferenceObject $(Get-Content $_.Group[1].Fullname) | Format-Table }