Apparently this works too:
Get-VMHostNetwork -VMHost (Get-VMHost -Name "Your ESXi") | Set-VMHostNetwork -DnsAddress 10.10.1.1,10.20.2.2 -SearchDomain test.com,other.com
and based on your point, this must mean that in this context stringa,stringb is treated as an array of strings, not as a simple string. If one puts quotes around the list of values it's rejected as an invalid parameter. I appreciate you pointing out this fine detail about the syntax of the cmdlet. I find the notation for it to be a bit obtuse -- not quite as easily obvious as BNF or other forms. The [] is too much like the optional parameter designation [ Parameter ] to easily set it apart in my eye.