There should be info on the joint ACA/CTA pathway on the CIOT website. It's not the same as doing ACA then doing CTA.
By mid tier do you mean top 10? What's the study support like? What happens if you fail an exam - resit at your own cost or is your contract terminated? Do you have to apply for a job at the end of training or is it continuous employment? Did you apply for a tax role or did you apply for general graduate intake and they selected you for tax?
As a minimum, I'd expect you to have paid leave to attend taught and revision courses as well as for the exam and maybe the day before. Some of the bigger firms provide additional leave in the week or so leading up to your exam. I don't know anywhere that would give you paid leave for resits (or retaking courses). The training providers offer pass assurance schemes so that if you meet certain criteria they will give you a free taught or revision course if you fail an exam. Some employers dismiss trainees for one fail, some for a repeated fail of the same exam, some for repeated fails across the qual, some only if your work performance does not progress. Check.
The joint pathway is a good opportunity.
Have you read the info on the joint pathway? I'm assuming not if you think it just means you have to do both quals simultaneously. It reduces duplication in the syllabus across the quals, means you take two fewer exams, and have more flexibility in your studies.
The combined info on what the specific aca vs cta exams are like plus how the pathway works should be more than adequate to gauge how you feel about it and whether it's right for you. There's nothing special about doing both qualifications (other than the doors they open), and arguably the joint pathway makes it much easier to be joint qualified than it otherwise would be.
If you're not someone who is good at seeking out information on your own and evaluating it then you might struggle if I'm totally honest. You need to be confident doing your own research to work in tax, so it's good to be doing that with your studies and comfortable locating the info you need from your professional bodies. They're really important skills. Similarly, juggling studying and exam pressures with your workplace responsibilities is also a skill you need for your career (it's not just pressure for the sake of it).
That's not trying to be nasty or put you down, but to help you weigh up whether it's right for you - I don't know the answers! The pattern we see with people who drop out of their qualifications is that the ones who expected us to tell them everything or do everything for them, right from how to register as a student, and give everything to them on a plate rather than looking it up themselves were the ones who dropped out.
Better to assess now whether it is the right fit for you than a year or two down the line.
How do you feel about registering as a student yourself without direction other than what you obtain yourself? Checking exam dates and deadlines to get yourself booked on for the right exams at the right time (it's not automatic)? Looking up the syllabus and searching for past papers? Learning to use legislation and research cases? Finding out what practical experience requirements you have to meet and evidencing them?
If that feels comfortable, then you should do ok. It is hard work, but not impossible if you put the effort in and have the aptitude.
If it is right for you, it's a great opportunity.
www.tax.org.uk/students-and-qualifications/chartered-tax-adviser-and-icaew-chartered-accountant-joint-programme