Sharing service apps between farms.

See Section 8

Run this PS on both farms and exchange certs:

$rootCert = (Get-SPCertificateAuthority).RootCertificate
$rootCert.Export(“Cert”) | Set-Content D:\Certs\ConsumingFarmRoot.cer -Encoding byte
$stsCert = (Get-SPSecurityTokenServiceConfig).LocalLoginProvider.SigningCertificate
$stsCert.Export(“Cert”) | Set-Content D:\Certs\ConsumingFarmSTS.cer -Encoding byte

Then you can use Central admin (Security->Manage Trusts) to enter these in.

From the Consuming Farm, run get-farm | Select ID to get the ID of the consuming farm.

$farmID = <ID from your farm>
$security = Get-SPTopologyServiceApplication | Get-SPServiceApplicationSecurity
$claimProvider = (Get-SPClaimProvider System).ClaimProvider
$principal = New-SPClaimsPrincipal -ClaimType "" -ClaimProvider $claimProvider -ClaimValue $farmID
Grant-SPObjectSecurity -Identity $security -Principal $principal -Rights "Full Control"
Get-SPTopologyServiceApplication | Set-SPServiceApplicationSecurity -ObjectSecurity $security