No dangers. Tagamet is the stronger inhibitor of the two so I don't think there's much point in using both. Though it definitely can't hurt.
The reason Tagamet potentiates (some) benzos is because it's a CYP450 liver enzyme inhibitor, the enzyme that metabolizes clonazepam (and a few other benzos). If you use an inhibitor the enzyme will get 'clogged', which forces more unmetabolized clonazepam to remain active in your system, causing a stronger effect. It'll also take longer for your body to get the clonazepam out of your system as it needs to be metabolized first.