Latch load limits

So I'm checking for my sister. Her son is 9 months and 23lbs. She has 2 Britax Marathon car seats. They are going into a Toyota Venza and a Chevy Malibu, what's the Latch load limit now?

You'll need to see what the manuals/labels for the specific Britax carseats state, then compare that to the vehicle manufacturer's limits, and then abide by the lowest LATCH limit stated by the carseat or the vehicle manufacturer.

GM vehicle models follow the 65 lb. maximum total combined weight of the child plus the weight of the carseat for LATCH installations, while Toyota states to follow the carseat manufacturer's instructions. If either of the carseats involved were manufactured since approx. early 2014, then the carseat's label should state the max. child weight for using LATCH to comply with the revised 2014 LATCH standard of 65 lbs. maximum combined weight of the child plus the weight of the carseat for using the lower LATCH anchors to install. If they're somewhat older Britax seats, they may state in their manuals to assume a 40 lb. child weight limit for using LATCH unless the vehicle manufacturer otherwise states a specific LATCH weight limit. (And if your head is spinning about all of this, sorry about that, mine is too from typing this up. LATCH has become an unpleasant quagmire of confusion despite its original intended purpose of simplifying carseat installs.)