Can't say I have mixed the two before but have had each separately on numerous occasions. For me personally It would knock me out cold for a good 12-16 hours!! Both are old school very sedating H1 antihistamines and for me are extremely
sedating, now that being said a mate of mine can have 50mg of Promethazine and hardly feel affected while the same does for me would have me out for 8+ hours, no possible chance of staying awake. Just of your info we are the same age, height and weight.
The only thing that sedates me more than the two above is Doxylamine, which does feel cleaner than the other two with less of a body load. Promethazine which I have taken at does up to 100mg was extremely uncomfortable, until I fell asleep, inducing muscle pain similar to opiate
withdrawal. Oh BTW these were taken due to a medical condition (bad allergic reaction) not recreationally.
I don't like your chances of a positive experience with this combo, But as I said in the beginning I have not done it myself.