I don't know how versed you are in pharmacology so I'll try and make sound simple:
Right so dihydrocodeine is metabolised into dihydromorphone by the enzyme CYP2D6 and further into nordihydrocodeine and other weaker metabolites (drugs) by other insignificant enzymes. Codeine itself is an inhibitor of CYP2D6, meaning that once taken, codeine will cause the levels of CYP2D6 in your liver to lower. Codeine is also metabolised by the same enzyme it inhibits, which is why re-dosing codeine doesn't work as effectively as taking one full dose all at once. So, as such, you should take dihydrocodone first as it does not inhibit any significant enzymes.
Unfortunately, dihydrocodone has a poor oral bioavailability, about 22%, so plugging it works a lot better. If you know how to plug, great, if not then read this:
http://www.bluelight.org/vb/archive/index.php/t-534901.html
If you're using a CWE to get your codeine/dihydrocodeine then you'll have to evaporate the liquid in the sun or in an oven on a low temperature to lower the volume of liquid. At most you can use about 5ml of water, I think when I plugged dihydrocodeine I used 1.5 but I had a proper syringe that I got from a needle exchange, you might have to use a CalPol syringe or something.
Yes, to answer your question, using both will make a better high, but 30mg of dihydrocodeine won't do much, honestly, especially if you take it orally. 22% of 30mg is like about 6mg, so plug it use a higher dose
