The Set-NaOption CmdLet -optionvalue parameter expects a string and it shouldn't matter if that's a comma delimited string containing multiple IP Addresses. I noticed that whilst the cmdlet thows an error it does actually set the option value for all servers so this seems like it could be a bug (IMO). It might be possible to invoke the API using "Invoke-NaSystemApi" but I checked the ZAPI and noticed this also fails using ZExplore from the SDK:
Ahh, from Matt's observations and my own experiments, this looks like a bug in the zapi.
The first time I try setting multiple time servers, the zapi reports an error but sets it correctly none-the-less. If I repeat the same zapi call one more time, the call returns success. Same behavior is repeated when using the PSTK cmdlets as well - since they internally call the zapi.
Another alternate (to Matt's) workaround is to run the Set-NaOption with ErrorAction set as 'SilentlyContinue' and then use the Get-NaOption to check if timeserver was set correctly. If not set correctly, the script should throw an error.