I would recommend not using AddScript if you are just invoking commands.
Assuming you want to invoke both commands but not pipe the results from the first to the second, and assuming you are using PowerShell V3, I would write the code like this:
powershell.AddCommand("Enable-Mailbox")
.AddParameter("Identity", ExchangeIdentity)
.AddParameter("Database", ExchangeDBName)
.AddParameter("DomainController", ExchangeDC)
.AddParameter("PrimarySmtpAddress ", ExchangePrimarySMTPAddress );
// AddStatement - like adding a ';' in a script - V3 only
powershell.AddStatement();
powershell.AddCommand("Set-CASMailbox")
.AddParameter("Identity", ExchangePrimarySMTPAddress)
.AddParameter("ActiveSyncEnabled", ActiveSync_Status);
Written this way (not using AddScript), you avoid one possible problem with script injection. If any of the arguments can come from a malicious user, it would be possible to construct an argument that closes one of the previous quotes, then adds malicious script and you'll run that script with admin privileges. Using AddCommand/AddParameter, you can avoid this situation.
Even if you control the arguments completely, you could still hit problems with some arguments if they contain characters like ';' and you weren't properly quoting the arguments.