Having re-read your original post, the fact that burning only one disc works only sometimes indicates that you should try to get that stable first, before trying to operate two writers at once.
Even when that is stable, I think any attempts to get two writers working at once on the same IDE channel are doomed.
I would stick with Nero 184.108.40.206 - 220.127.116.11 has various problems, and 18.104.22.168 is stable enough with my ND-2500A.
All I can think of is throwing out a few questions.
Does the speed test say your system is OK to burn at the speed you've selected?
Which IDE drivers are in use?
Are there any usable logs from a failed burn, or does it always involve a lock-up?
What happens if you simulate the burn rather than perform it?
What media are you using, and have you tried another brand or type (DVD+R instead of DVD-R, for example)?
What firmware have you got on the ND-2500A drives?