Question: Second Carry-on Bag on a Flight

Has anyone flown with Vueling with a carry-on luggage? According to their site, one other "smaller" rucksack/backpack/sling bag are permitted, but the size they described 35cm x 20cm x 20cm. That is really an odd dimension. Their criteria is they must fit under the front seat, and regular sized backpack should slide in without problem, although wider than described size. Has anyone flew with normal sized backpack/sling bag with a carry-on and had any problem?
I booked a flight for IdF (Rome - Grenoble, Vueling outgoing, Air France Return) wanted to save 80€ (and time) for checked in luggage, but I will have to be very creative... if anyone can give me some input about "second carry-on bag", I would be grateful!!

The second carry on bag depends a lot of the flying company. For example Air Canada and Westjet (big Canadian companies), allow 1 regular large backpack (that one that also has a compartment for laptop) plus a carry one regular bag (the ones with wheels for example). But cheap companies won't allow that. Anyway, always have with you all the electronics you carry and the necessary stuff for 1-3 days, in case the checked bag is lost. Cheap companies allow a purse or small backpack + the regular carry on bag. That's in Canada. I don't know if Europe companies are different

Thank you for the input @Sabrina In the meanwhile I did find there are "ad-hoc" bags being manufactured, with the exact measure 35cm x 20cm x 20cm which is the maximum of the approved cabin bags for the most low-cost airlines. They are not expensive and I decided it will be the safest option, low-cost airlines can be ridiculously fussy and I would like to avoid any risk of on-site last minute arguments and ending up having to check it in and paying for it just because the bag was 1cm too wide or something