I'm not sure if this is exactly what you are trying to do, but I was able to create a new group with two expressions using the NSX-T REST API. I'm using a direct rest call from vRO, but the principle should be the same for PowerCLI

var expression = [];
expression.push({
    "member_type": "VirtualMachine",
    "value": "tag-1",
    "key": "Tag",
    "operator": "EQUALS",
    "resource_type": "Condition"
})

// Not Adding a ConjunctionOperator to the expression list will make the API Call Fail
// The expression array index lenght must always be odd
expression.push({
    "conjunction_operator": "AND",
    "resource_type": "ConjunctionOperator"
});

expression.push({

    "member_type": "VirtualMachine",
    "value": "tag-2",
    "key": "Tag",
    "operator": "EQUALS",
    "resource_type": "Condition"
})

// expression.length == 3
var group = {
    "expression": expression,
    "display_name": "NewGroup",
    "description": "Testing Gropu Creation with multiple expressions"
};

var content = JSON.stringify(group);
System.log("Request Body is: " + content);

var restHost = System.getModule("com.domain.basic").getConfigurationElementAttributeValueWithPath("Domain", "NSXT", "restHost");
var restRequest = restHost.createRequest("PUT", "policy/api/v1/infra/domains/{yourdomain]/groups/NewGroup", content);
restRequest.contentType = "application/json";

var requestResponse = restRequest.execute();

Here is the result of the API Call in NSX-T

It should be possible (the Terraform provider supports it at least).

Make sure you have the syntax completely correct. Based on the output there, you'll need both expression AND expressions in your body.