Giving a lot of background, hoping it will help.
I am in Canada.
dd1 is 4.5. 40.5" tall, all in the legs. 33.5 lb.
dd2 will be 2 in the next few weeks. 31.5" tall. 23.5 lb
Our car: Mazda5.
2 Radians: one rear facing, one forward facing. Not a problem. No intention of switching anything around either right now. dd2 being such a light weight will benefit of another few months of rear facing in the main car. Not sure I will really have the patience to leave her rear facing until she hits the limit of her Radian rear facing, but not turning her yet.
MIL's car: She takes dd1 once a week and has a Eddie Bauer 3 in 1. Still harnessing dd1 in it. She still fits the harness, and the harness slot is above her shoulder. Taking dd2 very occasionally right now. I am ok with forward facing her in the Eddie Bauer occasionally like that. But Gma will probably want to take both once a week starting in September. dd1 still too light for a booster. Don't quite know what to tell her to get. (She got the 3 in 1 with her points and will not get an expensive seat like a Nautilus)
Our truck. That's the real head scratcher.
Ford Superduty Supercab. (It's a full with bench, but slightly narrower than the Crew cab).
dd1 has been riding in there about twice a week all winter. Currently in a 5 points to booster seat. The seat can be in harness mode up to 40" and 40 lb. She still fits it right now despite her height, just because she is all legs, not so long in the torso, but will probably still outgrow the harness in the next few months way before she hits the 40 lb mark to be in high back booster mode, given that she was 26 lb at 2, I don't expect her to be 40 lb until 6 or later. She has been riding either in the back seat or front with the air bag turned off. I am comfortable with the front given the size and nature of the vehicle and the fact that DH seems to get a better fit of a forward facing seat that way. My main issue with he front is that if I ride too I don't want to ride in the back when we go as a family lol. I am not 100% sure if she has the maturity to sit properly in a booster either but I think it is very possible that she does for short rides. She does not sleep in vehicles.
We were only taking dd2 occasionally in the truck and moving her Radian from the car to the centre of the back seat of the truck, but it is a real PITA. And with summer around the corner, we will take the truck more to go for bike rides and camping trips. I would like her to have a permanent seat there. Did a little reading and I think with the risk of hitting the rear glass so close RF, I am starting to prefer dd2 forward in the truck. She is two after all. My initial though was to put dd2 in the harness to booster convertible in the centre and get a Nautilus (or Frontier?) for dd1, but DH doesn't think he would be able to get a good install with the Nautilus: tethering a higher shell in the back will become nearly impossible. The lack of space in between the back seat and wall combined to a higher seat will make it a 90 degree angle down to the anchor point. It is not going to help much to tether it effectively. And his point that an an untethered 5 points is no better than a booster ...which I see the logic but the kid is not there weight wise. :/
Oh and we never got a Radian to fir very nicely forward facing in the truck for some reason.
Would car seats gurus have other suggestions? WDYT?