I had 2 kids, in various combos of a Graco HBB then a booster cushion and infant carrier (Britax shr ii) then group one (mothercare madrid) and a size 14 lady in the middle of them in the back of a Kia Rio 2010. I also had an oyster max double in the boot, and weekly food shop and a springer spaniel. That thing was a tardis and I loved it. It was also in a 6 car pile up on a dual carriageway and was fantastic. I gave it to Sil when dp got a company car and it’s still going strong for dneice and dnephew with 2 ERF seats in.
Renault scenic and one of the Citroen Picasso things were the 2 we found with full seats in the back when we were looking. In the end we bought a 7 seater (sharan) so we have the 3 full seats and massive boot space too.
I have a Citroen Berlingo with a rear facing baby seat in centre and isofix hhb on either side . I can get into middle if I take baby seat out and am reasonably comfortable. I also love the sliding doors . The boot is big enough to put the pushchair in without collapsing it .
I’ve got a citroen c4 picasso and can easily get 2 ERF car seats in the back and someone in the middle. All 3 seats are full size and isofix so could technically get 3 car seats across the back. The boot is massive too, I fit our double buggy and a pushchair in there with space for shopping. We even managed to pick up 2 wardrobes from Ikea and still had room for one car seat.
If you aren't big it's doable for a long journey with 2 RF (2 way elite and infant carrier in an 09 Mondeo. Not convinced it would be comfortable for more than a short trip if 1 or both kids forwards face because shoulder room would be more problematic than bum room - I'm a 28 inch underbust and shoulders were definitely in range of DD's feet