adding multiple products in a flexible cart is not possible because the product names are not taken in consideration, see Widget.js line 76:
REPLACE:
if (product.currencyCode) {
params['currencies[' + index + ']'] = product.getCurrencyCode();
}
index++;
BY:
if (product.currencyCode) {
params['currencies[' + index + ']'] = product.getCurrencyCode();
}
if (product.name) {
params['names[' + index + ']'] = product.getName();
}
index++;