My PCP gave me a prescription and it's for 100 mg pills.  I am not a big guy, 5'7' and 135-140 lbs, so I never felt comfortable taking a whole pill.  Half a pill works just fine for me.  I've read that the dosage is once per day.  I am contemplating an overnight with a favorite lady.  So we'd be engaged in activity that evening and again in the morning which is less than 24 hours.  Does anyone have experience with two doses in less than 24 hours?  I know one of the answers will be ask my doctor, but I'd rather not have that discussion with him.

.... I did stay at an Holiday Inn Express last night!

You say you are taking a half-dose currently.  So if you take two halves in less than 24 hours, then you will equal one which is what YOUR doctor prescribed so should be fine.  

Everyone is different.  Splitting the 100 mg is smart.  I think you will be fine taking 50 mg 8 hours apart.
My sweet spot is 40-60 but I take 20mg pills.  Sometimes one 20 mg is enough and I feel I can pop them every 4 hours or when I feel I need it.

Once per day is based on taking the pill at the strength recommended by your doc, which you say is 100mg.
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Clearly if you take 50mg one day and it is mostly gone from your system, then if you take 50mg the next day, the dose never exceeds 100mg.  So I am not sure I see the issue.

That’s why I prefer Cialis. It lasts 24-36 hours.

Hi. I, too, have been prescribed the 100mg dose. And I've broken them in half and taken 2 halves spread out over the day and haven't had any problems because of it. I can't be sure that it'll have a different effect than all 10mg at once. But it hasn't caused any problems.

Pharmacokinetics, half-life, is how long the medication stay s in the blood. Sildenafil (Viagra) has a half-life of 4 hours and vardenafil (Levitra) has a half-life of 4-6 hours while tadalafil (Cialis) has a half-life of 17.5 hours.

So, if you took one-half tablet (50 mg) to start, this is how much would remain in your blood after 4, 8 and 12 hrs.  
50 mg - 0 hrs.
25 mg - 4 hrs.
12.5 mg - 8 hrs.
6.25 mg - 12 hrs.

I can definitely feel the effects of viagra the morning after, which could be anywhere from 9 to 12 hours,

I can definitely feel the effects of tadalafil 10  mg after 48 hours.

Feel free to ask your doctor why you got 100mg, but some prescribe 100mg because they often cost very little more per pill than the 50mg, so you save quite a few bucks.  

Back when Viagra was crazy pricey, it made a lot more sense to cut the pills in half, and that's what my doctor at the time recommended when I weighted about 250 at 5'10". This was like 10 years ago and the pills were like $40 per.  They do make pill cutters for just a few bucks you can buy at just about any pharmacy or online, and some are designed for specific shapes of pills or are universal.

In recent years I've been taking Sildenafil 20mg -- usually 3 pills (60 mg) and that tends to work fine for me.  80mg is a bit more than I probably need, and if I feel brave and take 100mg and I get really flushed and my nose runs annoyingly.  

So I'd stick with the 50 mg you take now, and if you think you need another 50 mg the next morning, go for it!