100mg is the maximum daily dose of sildenafil. Even though you're not supposed to take it more than once per day, taking it more often should be OK if you don't exceed the 100mg maximum, I have done this many times. Taking two 50mg an hour apart helps reduce the stonking headache that 100mg gives me.
Cialis would be better for that, as it lasts longer. Bear in mind that the maximum dose for cialis is 20mg, so if you're tempted to mix the two, multiply your cialis dose by 5 and stay within the 100 limit overall - eg 10mg cialis and two 25mg sildenafil.