There is no perfect solution. Not on measurement and also not regards rewarding, as each individual person has his own values and interpretation of "fair". One person may have no issue to work long hours, one may value flexibility more, one may bring more knowledge and one may bring more network. Same goes for rewards... one may depend more on salary.

From my point of view, there is only one option... sit together and learn about values, evaluation (which each does differently, due his personal values) and find measures which fit for both (it will be a compromise!). Maybe you even find only rules for a certain time/phase. And if something changes you need to sit together again. And don't forget there are at least 3 parties if you are 2 founders: The two founders and the company! And because of this, I think you should do a vestment agreement and define some basic rules (e.g. what if one has to work less, because the startup does not make revenue yet and he needs a second job? What if one wants to leave?) I promise you will learn a lot of each other - independently how long you know each other already...)
Does this sound complicated? Yep, probably... but somewhere I read that the average life-cycle of a startup is longer as the average of a marriage. And like with a marriage, you need to work on the relationship / partnership. I fully agree with what Pedro Machado said ...