There is an obvious answer here, and some undercurrents as well.
The obvious answer is that people should be paid 'what they are worth' - whether their pay is in normal salary or earned equity or in any other form.
The undercurrents :
'Founding partners' is an amorphous concept and while the concept of 'equal' may be implied, it seldom occurs in reality. Maybe one partner has the idea, another has the time to execute it, and a third has the money to fund it. Those are three very dissimilar contributions and there is only fairness in treating them differently.
If a founding partner walks away from a job paying $200k elsewhere, that doesn't automatically mean he is entitled to the same pay in the new venture, of course. That is an added level of complication - should you pay your founders at the same level they were getting elsewhere, or at the rate which other people would cost to do the same job in the new startup.
The big thing is to realize that there is no obligation at all to have all founding partners paid the same, and to discuss and agree between the partners on what is fairest, as part of the upfront partnership discussions.