I try to get people in this situation to evaluate what each is bringing to the party. The 3 factors are:
Money
Time and effort
The concept or vision
If it is a joint concept, and both couples are putting in equal time and effort and money, a straight 50:50 split is sensible. The problem arises where some of these are not equal.
You indicate that both couples are investing the same amount of money, so you can ignore that from the "fairness" calculation.
Whose concept is it? Do both couples accept that you jointly "own" the concept? If so, ownership of the business should be 50:50 and that means 50% of the profits - after a fair "salary" for the time and effort.
So you need to discuss and agree what your time and effort are "worth" to the business. Don't forget that you will put in time over and above the actual normal working hours, which probably cancels out any time the other couple put in in terms of advice and support. Having agreed what is a "fair salary" for what you will do, that should become the first tranche of the profit and any shortfall is rolled forward into the next year and so on. Only when you have been paid for your time and effort at the agreed rate should there be any question of a share of the remaining profits.
You might think about a limited company for the business - it has various benefits anyway, and you might all find it clearer to think in terms of you and your wife being employees and therefore receiving a salary (albeit that some of that salary may have to be deferred until the company can afford it) and both couples are shareholders, who will get a dividend only if there is a profit after paying the salary.
As Alan says, get a partnership agreement drawn up (or a shareholders agreement if it is a limited company) by a solicitor - I have dealt with a number of cases where partners have fallen out and it all gets nasty - but they were the best of friends when the business started.
In fact, you may find when you discuss the value of your time and effort, that the two couples have very different ideas about that value - better to fidn that put now, than in a year or so's time when you are expecting to get the income you think you have foregone in the early stages, and the other couple don't think you are entitled to any!
Hope that helps
GJ