5 Hour Energy consists of B vitamins, amino acids, and an unspecified amount of caffeine (presumably around 200mg). Too much of anything is bad for you and I wouldn't drink one of these things every day (mostly because of all the B vitamins), but I don't see how adding 2 of them to 3 gallons of beer would be harmful. The only danger of combining the two drugs that I can think of or find is dehydration due to the increased diuretic effect. Sure, you'll have a mildly increased diuretic effect combining the two, but we're only talking about roughly 12.5mg of caffeine per 12oz beer. Highly unlikely to have ANY noticeable effect. All of the dramatic news stories are related to BINGE drinking and much more caffeine than we're talking about.
Still, it's probably a bad idea for anyone with significant cardiac disease and what's the point? Caffeine does not counteract the sedative effects of alcohol very well, especially at these dosages, and it definitely won't give you any extra "pep". There's better/tastier ways to add berry flavor. If you want extra B vitamins (which hardly anyone needs unless they have a medical condition that causes malabsorption or chronic alcoholism) take a multivitamin a day.
Edit: The comparison to speedballs = ridiculous. Might as well compare a BB gun to a Howitzer.