Thanks stehk, using 2 dictionaries at the same time works perfectly on my Kindle 3 now! My use case is slightly different: Dutch-English dictionary as primary and English-Slovak dictionary for case I do not know an English word from the Dutch-English translation (there is no good direct Dutch-Slovak dictionary).
At first I thought I would set the language of the primary dict to en, so that I can keep correct dict metadata (en->sk) for the English-Slovak dict. However, from some reason this did not work - the primary dict was always used... so I tried your example of setting the primary dict to fr and the secondary to fr->sk and that worked. I don't understand why I have to work it around in this way but not a big deal for me