Oi, sorry, yet another long reply but a bunch of thoughts.
Credit: you need to build your own when we're talking long term. It might be years or decades even from now but eventually nobody will be using the current mortgage trifecta and when that happens you can basically kiss your scores goodbye if they're based on AU's. They may count, or they may not, and speaking to a mortgage loan agent who just pulled your credit is not the time to find out that it's a problem: that's too late to fix the issue and your mortgage dream might just hit a one to two year reset at that point. That would well and truly suck just sayin'.
Also you're already thinking of split credit with shared financing: if you get into financial trouble as a family you can tank one score but not the other and still have access to the modern credit market, which I'd suggest is a good thing when it comes to financing. I'm not a fan of AU's beyond the building credit stage, one event could wipe out both reports. Credit is a resource for smoothing out financial decisions and two separate credit files is nothing but a win compared to effectively one shared credit profile.
Fact is lenders hate AU's, they wanted FICO to get rid of them for years, didn't quite work out that way but nobody is clear how FICO 8 is using their anti-abuse and we can certainly assume FICO 9 is using something similar or worse. If you get to a manual review at best your AU's won't help you, I'd argue CGID isn't quite right in this case (and that's a rarity) they might actually hurt you. As a result I'd strongly encourage anyone to build their own file to where the AU's are irrelevant for scoring purposes.
Debt: That all said, focus here first. We talk a lot about credit on this forum but frankly finances >> FICO, always.
An installment loan if it's at a lower APR than your credit cards is an absolute win both financially and for FICO, but if it's parity it can help FICO, but if it's worse never ever do it.
Shared accounting on credit reports: if it's on your credit report it counts as an obligation by the fact that it could become your obligation so any debts on both will count towards DTI for both. In joint apps like a mortgage where you have a heavy manual component in underwriting that's probably seen if you're applying jointly for using both incomes and probably doesn't double-count, but if you apply singly it absolutely does even if you haven't made any payments in 3 years on a 5 year note.
Ultimately stopping the bleeding due to interest payments is crucial: I'm at the point in my life now where I'm throwing all available free cash flow at a 5.25% debt even, and that's way different from a standard rewards credit card which probably averages around 18% APR now.
If you have time before your next big ticket purchase (auto loan or mortgage) then work on your credit now which means your own credit cards (and maybe strategically dumping the not so great AU's from a limit / age perspective) and if you can swing the personal loan at a better interest rate to pay off the debt, go for it.
