For youngest:
I think I'd keep the Joie Tilt since it won't be used that often, makes sense not to spend if you don't need to.
But if you did want a spin seat, the Graco Turn2Me is basically the same as the Joie but is cheap. Or you could look at upgrading your main seat to something like Britax Dualfix Pro M (higher weight limit, better leg room) and moving the Joie seat to the other car.
For eldest:
Which Britax seat are they in now? I'm guessing a multi stage one like Evolva or Advansafix? I would say best to keep them in a harness until closer to 18kg if possible, but at 105cm they are likely to be close to the limit of any 18kg harness anyway.
You could look at a high backed booster seat for the other car, if you have money to spend on it, it could be the one that becomes their main seat later on. There are great options from all the usual brands. Or if you wanted something that can be portable for travel, Chicco Fold & Go is a good option to look at - but this seems out of stock everywhere at the moment. Or just a budget one. TBH the price of HBBs have sort of collided, so the "good" budget models are around £100-150 while the best rated ones are still capped around £180-200, and you can generally get some great mid range options for £120+. Try out in a shop if possible, and stick to the known good brands - Cybex, Maxi Cosi, Britax, Joie, Nuna, Recaro etc.
Or if you wanted another 123 type, you'll want a non-isize - you could look at Britax Advansafix 3 (easier to find than the 4 somehow) or Britax Evolva SL SICT. Recaro Tian has a good safety rating, but I don't know how high up the harness goes on it.
Or it might be worth getting a good quality Group 1 only, FF type seat like Maxi Cosi Tobifix, use for DC1 for now, give to DC2 when DC1 outgrows and change DC1 to high backed booster at that point - just for a cheaper option but keeping the harness.