In my opinion, i think a double pedal is definetly a better idea. For these simple reasons:
1. 2 dissimilar bass drums will sound weird. Double bass is meant to be a like a constant snare roll, but on a bass drum, and you will get 2 different sounds from drums made out of 2 different woods. (but experimenting with different sounds could be interesting)
2. If your gonna invest in something expensive, dont waste it on something not perfect.
3. You cant use double bass yet, and you will not get anything more out of learning on 2 drums rather than 1 drum and a double pedal.
4. Anouther bass drum is yet something else you have to move around when gigging!
5. Buy a double bass pedal, and spend the rest on sticks. You'll get a lot more for your money.
Just some thoughts...
