Sorry to destroy your great suggestion here Quall but I would recommend to NOT use the X2 5035 bios on a DuoX2... There have been several problems reported using that bios on a DuoX2. Some had luck and got it working other found themselves in a tough position and had to get a programmer to reprogram the flash to get it started again...
Personally I like the iND bios series... Especially the iND-5003beta, that simply makes my day...
Works like the X2 5035 bios with a config file but is only 256kb instead of 512kb, thus you can have a very reliable backup bios on the other bank (like EvoX M8 or similar). If is is an 1.6 xbox I would say get the EvoX M8+ bios and use that.
If it is not get iND or the X2 4983 (tough I prefer 4981 as they are editable).
Other than that I think the DuoX2 is a great chip and comes at a small price.