[SalesForce] How to pass multiple parameter in URLFOR in Custom Button JavaScript

var iframe_url = '{!URLFOR("apex/dummy_VFP?id="+ customObject__c.Id)}';

The above one is working perfectly, but when I tried to pass one extra parameter its giving up syntax error, below I tried:

var iframe_url = '{!URLFOR("apex/dummy_VFP?id="+ customObject__c.Id&buttonType:"conf")}';

Please suggest on above.
Thanks in Advance.

Can you try this.

var iframe_url = '{!URLFOR("apex/dummy_VFP?id="+customObject__c.Id+"&buttonType=conf")}';