That's what I was thinking. Why would you need the viagra?
The OP doesn't state his age, but if he's going to a rave then chances are he's somewhere south of 30. Barring some fundamental ED problem (which he doesn't mention), there are two reasons why he'd plausibly want to combine X with Viagra:
1) The X interferes with his ability to achieve an erection. In my experience, X and sex go together perfectly well: I never had a problem. But that was when I was in my early 20s
2) He either wants to show off a continuous hardon or expects that the rave will degenerate into some massive orgy.
The only other explanation is simple curiosity. If that's the case, then it's his life to live as he sees fit, though it's nothing I'd do, myself. Viagra effects your blood-pressure; combining it with whatever is being packaged as "X" (not the MDMA of my day, probably) can lead to any number of potentially lethal side-effects.
I'm not a prohibitionist (at all), and I'm anything but risk-averse when it comes to squeezing the most hedonism out of life as you can, but this just seems kinda foolish, IMO.
It's both the prerogative of youth to engage in foolishness and a prerogative of age to point such things out as folly :wink: