Can I use both the LATCH system and the vehicle seat belt at the same time?
No, you must never use both the vehicle seat belt and LATCH belts at the same time when using the car seat rear- or forward-facing. Both installation methods are equally safe, but only one should be used at a time. Review section 8 and your vehicle owner's manual for proper installation guidance. (Page 7, 23)
From the official Graco Graco Extend2fit Convertible Car Seat User manual, page 7
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