The easiest way is actually to send all of the bottom ending numbers[0-4] to one account and the top half ending numbers[5-9] to the other account. You can do this through some pretty simple dialplan logic:
; put this in the first carriers dialplan entry
exten => _91NXXNXXXXX[0-4],1,AGI(
agi://127.0.0.1:4577/call_log)
exten => _91NXXNXXXXX[0-4],2,Dial(${TESTSIPTRUNKX}/${EXTEN:2},,tTor)
exten => _91NXXNXXXXX[0-4],3,Hangup
; put this in the second carriers dialplan entry
exten => _91NXXNXXXXX[5-9],1,AGI(
agi://127.0.0.1:4577/call_log)
exten => _91NXXNXXXXX[5-9],2,Dial(${TESTSIPTRUNKY}/${EXTEN:2},,tTor)
exten => _91NXXNXXXXX[5-9],3,Hangup