How can I validate more than one checkbox on submission?

I have two sets of checkboxes, named pizzaBase and pizzaTopping. I want to validate my form so that on submission at-least 1 base and topping must be selected.

function main() {
    var form = document.getElementById('formname');
    form.addEventListener('submit', validate);
}

function validate(event) {
    var form = document.getElementById('formname');
    if(!form.pizzaBase.checked) {
        alert("select a base");
        event.preventDefault();
    }
    else if(!form.pizzaTopping.checked) {
        alert("select topping");
        event.preventDefault();
    }
    else {
        return true;
    }
}

Problems

1. Only first if statement works.
2. Form submits true if checkbox is checked and then unchecked before submission.
3. Doesn't validate, doesn't work.

Where am I going wrong?