I'm not a physician, but from what I do know is that mixing the two can increase cardiac stress and has led to death in some cases.
Cocaine has a very short half life of 1 hour. Conversely, the half life of amphetamine (not including closely related compounds) lasts 12 hours.
Like you planned, I think that using cocaine beforehand provides the opportunity to sober up completely sooner than compared with amphetamine. I wouldn't use amphetamine within 4-6 hours after ingesting any significant amount of cocaine to allow its pharmacological action to be decreased, in effect reducing risk of damage.
From what I remember both work on similar physiological actions so I imagine one could spoil the psychoactivity of the other giving you even more reason to space out the use of either drug.
Don't indulge too frequently and stick to reasonable doses. Listen to your body before ingesting the second stimulant and you should reduce serious complications IMO.