What practical speeds in KBs can I expect from my dual ISDN service?

The ratio Kbs vs. KBs should be considered around 12 for practical purposes of data communication, including all the excess bits needed for synchronization, error detection, error recovery, re-transmission etc.

Many data communication services ( like modem , ISDN, T1 ) are rated by the bit throughput rate Kbs ( kilo bits per second ). At the same time at application level the data throughput is rated by byte throughput rate of KBs ( kilo bytes per second). In order to compare those two you've got to know how many bits are in one byte.

The common wisdom is 1 byte = 8 bits. This is true from software standpoint, however it is not true from a data transmission standpoint. Extra bits are being added to secure synchronization, error detection and recovery. Furthermore when transmission error happens at the low level data is being automatically re-transmitted, without this being visible to the higher application/software level.

All of the above can cause the transmission speed reported by your software application in KBs to be lower than your transmission circuit speed in Kbs divided by 8. Tis is normal and it should not bother you.

9.5KBs to 11KBs is an excellent practical range for dual ISDN channels with maximum speed of 128 Kbs.
4Kbs to 4.5KBs is an excellent practical range for 56 Kbs modem.

Do not forget also that the speed you get depends on other factors as well like the network throughput, the capacity of your source to provide the data etc. It is not unusual to get mere 20 KBs throughput from your T1 line ( 1,538 Kbs ) if the source of data does not have enough bandwidth available.