Best timing for credit card application

Background: we took out a new mortgage product two months ago because the old fix had come to an end - we're now fixed for 10 years. We took an additional £40k of borrowing to pay for some major house renovations. This was wildly within our affordability - they would happily have loaned £100k more than that, and we'd easily have been able to make the repayments. I made it very clear in the application process that £40k was a guesstimate and that we might need to borrow more. We've now got accurate quotes for the work we want and the total bill is going to be £67k. I have £10k in easily accessed savings, so went back to the mort company for £17k additional borrowing - only to be told that their policy is that they will not permit additional borrowing in the first six months.

I've made a bit of a fuss about the mortgage advisor not telling me this - apparently he was supposed to and is undergoing some 'retraining'. But they won't bend the rule and will not make a further advance for another five months.

So we need to access another £17k. I applied for a 2-year 0% on purchases type credit card and was immediately given a £7k limit. Our thinking is for my wife to do the same, and for me to apply for another one. I'm pretty comfortable about taking this debt on; as things stand we'll probably be able to repay it within the two years, and if it looks like we won't then we'll easily be able to take on the extra mortgage product once the six month embargo is over.

My question: when should we make the two credit card applications to have the best chance of succeeding? My instinct is to wait for as long as we can, so it's not hot on the heels of my first application last month. And also that we should each make an application simultaneously, to two different companies, so that each gets approved before the other one starts to show up in credit scoring - I can see that two of us making applications one after the other might look a bit desperate! Does this sound like the best plan?

(If it all goes completely wrong, I do have a never-used credit card with a £10k limit (but only the usual 56 days interest free), so if we cannot get further 0% cards I can at least pay for the supplier, and I guess my ultimate backup is to apply for a non-secured loan or to cash in savings that come with early withdrawal penalties.)