Yes you can, this is how I do it.
I have RASPPPoE installed on my network card (the protocol you need to use Route Sentry).
Create a "dial-up" connection for each account, dial them both (ensure that the number of WAN endpoints is at least 2, how to do this should appear in any good Route Sentry guide), then create a little batch file (in notepad, just save as something.bat) with the following:
route add 196.26.208.123 mask 255.255.255.255 IP1
route add 196.43.2.61 mask 255.255.255.255 IP2
Where you replace IP1 with the IP address that you have on the IS account and IP2 with the IP address you have on the SAIX account. The IP address at the beginning of each line is the IP of the respective news server.
Save the BAT, run it and you should be good to go
If you want to combine this with an international account (3 simultaneous connections) it does get a bit tricky as you need to edit the Route Sentry routing file (localroutes.dat) but that too is possible
