Someone asked something similar on my build log (https://bitcointalk.org/index.php?topic=255582.20 post 37). I was doing a comparison between btcguild PPS vs btcguild PPLNS vs slush (better with slush I thought after a week). For my setup on my raspberry pi I had it to run three instances of CGminer each using different block erupters (2 each), so I had different -S tags in the script that would load up cgminer automatically on power up (but on PC you can just put this in your .exe command to run), and then each using a different .conf file with different worker and server settings. All instances worked perfectly, although there was a warning about the API being already in use but this was a non-fatal warning and did not effect mining. I am assuming the BFGMiner is written very similarly to CGMiner so a first job would be to check this but quite possible then