It wasn't (Crossblooded didn't exist when Ashiel wrote the Domain Sorcerer option), but the text prevents them from working together in any useful way since it completely shuts off all bloodline class features.
I'm inclined to agree with that, because combining the two negates most of the weaknesses of Crossblooded (or all of them if you pick the right domains) and produces a raw boost in power above what either of them provide alone.
Effects of Crossblooded
- Extra class skill from 2nd bloodline
- Can pick bloodline spells from either bloodline (you still get one per spell level)
- Larger bloodline feat list
- Extra bloodline arcana from 2nd bloodline
- Can select bloodline powers from either bloodline (you still get the same amount of bloodline powers)
- 1 less spell known at each level
- -2 to Will saves
Effects of Domain Sorcerer
- You don't gain a bloodline class skill or arcana
- You gain no bloodline feats
- You gain no bloodline powers
- You get 2 domains
- You gain 2 extra spells known of each level from a fixed list
- You gain 2 sets of domain powers
Effects of Crossblooded & Domain Sorcerer
- You get 2 domains
- You gain 2 extra spells known of each level from a fixed list
- You gain 2 sets of domain powers
- 1 less spell known at each level
- -2 to Will saves
The loss of a spell known at each level is offset by getting another 2 from your domains without losing bloodline spells, and the penalty to Will saves is outweighed by the effects of domain powers (some of which boost saves). If it worked, it would be strictly superior to both Domain Sorcerers and Crossblooded Sorcerers.