I'm surprised you guys know so little about the combo. It's a favorite on 420chan. I've never heard of anyone dying on it or experiencing serious side effects (delirium excluded). I know this thread is from 2010 but I'm just gonna throw this up for anyone that reads it and is curious:
THIS COMBO IS NOT SUITABLE FOR THE INEXPERIENCED IN ANY WAYS. Honestly, I don't recommend anyone fuck with DPH, it is toxic to both the brain and heart and can cause all sorts of awful issues such as heart arrhythmia (short term), extreme memory loss, anxiety and HPPD. The HPPD and anxiety can last months or years.
I've never done the combo myself but I've talked to enough people who've done it and read reports from those who enjoyed it. The draw of the combo is that they are both metabolized by the same liver enzyme and thus they potentiate eachother. In theory the DXM body high (should) cancel out the DPH bodyload while the DPH visuals should manifest with far less fear-filled delirium due to already being high on the DXM. The result is less relaxed than DXM, but with strong OEVs and a light but (hopefully) pleasant delirium. The usual recommendation is 300 mg of each. While this will certainly get you fucked up the DPH apparently overpowers the DXM at these doses and the experienced suggest keeping DPH at half DXM, or at least 200-300 mg below it. for the more experienced 300 mg DPH to 500-600 mg DXM would be very intense but also very rewarding. On the lower end 200 DPH to 400 DXM should provide all the desirable components of the high. 100-150 DPH+300 DXM would probably be a safe bet, but it might not be enough DPH to get clear open eye hallucinations. It would still potentiate the DXM a great deal though.
Take the combo at your own risk.