Or I don't know why but it does happen with my A4000's IDE. In fact, the CF card doesn't even work with a CDROM on the same cable. This is a Sandisk 4 GB.
Its either a problem with the Adapter that you use, or a problem with the CF Card.
I have 3 devices on my A1200 IDE for 13 years without problem but I am cheating and using an IDEfix97 4-way adapter thingamajiggy to get faster speeds.
Countless ppl use 2 drives on A4000s and A1200s.
Its just that as soon as you try to use a CF card your chances of having problems go up dramatically. Lots of ppl can't even get 1 CF Card all by itself to work. Lots of strange bizarre problems. I have read many threads where it is stated that many CF cards do not 100% conform to IDE specifications.