Very very bad advice when it comes to harm reduction. Cardiac issues. Don't do this if your life is precious to you!
E.g.
http://www.ncbi.nlm.nih.gov/pubmed/21796081
This is a relatively common problem when combining PDE-5-inhibitors with monoaminergic stimulants.
The "risk" of this combination is frequently cited, but I've never seen it sufficiently supported. Your link itself actually suggests that it is the first case report of a complication from the combination of sildenafil, amphetamine, and alcohol.
Personally I'm not convinced. Consider that the first line of treatment in a medical setting for hypotension associated with pde-5 inhibitor overdose includes vasopressors like dopamine or norepinephrine. Consider also that pde5 inhibitors are commonly administered as a treatment for pulmonary hypertension.
There is also no medically recommended contraindication for pde5 inhibitors with prescribed stimulants like adderall, Dexedrine, etc. In fact it's relatively common in the United states for doctors to prescribe viagra alongside a prescription stimulant, if erectile dysfunction emerged as a side effect.
---To be fair, we're talking about medicinal vs recreational doses here, so this doesn't necessarily suggest that there is no interaction but rather that any interaction that exists confers little risk at an appropriate medicinal dosage of each drug. Whether or not this risk remains small at recreational doses of the stimulant is an open question. Given that stimulants decrease perfusion through vasoconstriction, while pde5 inhibitors decrease perfusion through vasodilation, it doesn't make much sense that their negative effects on perfusion could be additive.
Basically, I can see how a stimulant could worsen perfusion in a hypotensive crisis (via worsening tachycardia and subsequently decreasing ejection fraction), but I can't see how this would occur absence of extreme hypotension. It seems much more likely to me that your case report describes an interaction between the alcohol and the pde5 inhibitor (hypoxia from respiratory depression and low perfusion from low blood pressure), while the amphetamine served to worsen its consequences.
If you've seen significant research to suggest otherwise (ie. To demonstrate/support a mechanism, rather than just concurrent use), I'd love to see it, because its been bugging me for a while that a belief in this interaction is accepted so readily in drug communities yet is so absent in the medical and scientific community.
Personally, I would assume that low doses, sufficient to relieve stimulant vasoconstriction, should be relatively safe, while doses intended to have a pro sexual (ahem, pro bono I mean pro boner) effect might be more risky.