in the current scenario you have to use 2 queries to achieve this.

But if functionally its alright, i think you can use OR query and achieve the same thing in 1 query only.

var bills = new GlideRecord('u_citnet_billing');

  bills.addQuery('u_service_billing_profile.u_fund_to_credit', 'CONTAINS', 'NF').addOrCondition('u_service_billing_profile.u_fund_to_credit', 'CONTAINS', 'IN');

  bills.addQuery('u_billing_status','Ready for Billing');

  bills.addQuery('u_billing_account',account);

  bills.query();

  while(bills.next()){

  //perform some calculations

  }

Try the code above