Question on tadalafil dosage

I have a question.

Normally I take half of a 50mg sildenafil and it works great. I also have some 20mg sildenafil plus 20mg tadalafil pills. I got them because they supposedly last 36 hours and many times when I take a sildenafil only things don't work out and I wind up not having sex (wife falls asleep) so I wanted something that lasts longer for those occasions so I don't waste a pill and take one the next day.

Anyway, the 20mg + 20mg is too strong and gives me a headache the next day so I take half of one of those also. Works fine that night, but I am wondering if that should still last till the next night since the sildenafil does not last so that leaves me with just the 10 mg tadalafil. Should that still work 24 hours or more later or should I boost it with half a sildenafil?