I can't answer your headline question (about propotion of income), as DH and I have never fully merged our finances, so I don't know.
But you need to think carefully, and look at your DCs' education as a whole - and in a series of steps. Paying for prep school may be the most important step, in terms of setting the DCs' early experiences of, and attitudes to, school. But what about education after that? Do you have state secondaries near you that you would be happy with? Or are you looking at affordability to 18? Might it make more sense to re-examine the state options, and save up for the later years? These are personal questions which can only be answered in terms of your views on what makes a good education, and what the realistic options are for you.
Your job security and career expectations are also factors.
Also, you need to look at how much the school really costs, not just the headline fees. Are lunches and trips included? What clubs (eg ballet) are billed seperately? How much does the uniform cost? How much does late room provision cost? Then remember that school fees inflation has outstripped other indices for many years - allow 7-10% uplift pa. How do the sums look now?
Many private school parents are on middle incomes - if you do go for it, you will be far from alone in having to think hard about family budget choices all the time.
Personally, I'd go for it whilst you can if you know it's the right school for you - who knows what tomorrow will bring?