We've flown Virgin in economy several times, with a couple of wheelie bags (in the overhead compartment) and a small duffle bag/backpack under the seat. I would be 99% certain that as long as your second item fits under the seat in front of you it won't be a problem. They just don't want people bringing on a full second suitcase and claiming it's "only a backpack". And even then, if it's not a full flight, I've found Virgin to be very kind and accommodating. Count my vote as "fine, I do it all the time".