Ditto PPs that you can't use both at the same time. The seat is crash tested using either/or and using them together and in an accident using both at once may put too much stress on different parts of the carseat, which can actually cause failure.
I would definitely try to go to carseat inspection. Check car-seat.org to see if they have a tech on their list in your area. Fire stations and police stations often have someone on staff who is not up-to-date on their information and can cause more harm than good (I've had friends yelled at recently that they must turn their child around because they're 1 year and 20 lbs, which is absolutely no longer the case).
Also, when using LATCH, check in your carseat owner's manual if it's allowed where your seat is installed. Very few cars have LATCH for the middle seat and/or allow you to "borrow" one LATCH anchor from each outboard seat. Most of the time if you use the middle seat you must use seatbelt, so read your car owner's manual to check that.