Question regarding taking sildenafil AND Tadalafil together

[Never_Enough]

Something I've never been able to get a clear answer to on Reddit and other places because people seem to misconstrue the question.

Every online and basic medical resource seems to indicate you shouldn't mix both pde5s.

However they also say that other than duration and onset they work in the same way.

Given this to be the case why should taking 50 mg sildenafil and 10 mg tadalafil be any different to taking 100 sildenafil or 20 mg tadalafil?

Bearing in mind I'm not talking about taking both at maximum doses or taking over what woukd be the maximum dose of one combined, but taking a combination like 25 mg sild and 15 mg tad or 5 mg tad and 50 mg sild.

Is there any reason why it should be more dangerous than taking the full dose of one?

Thanks.