1 If you assign the ip pool address to your PPTP client is part of your inside network ( for example : you inside network is 192.168.100.x/24 and your PPTP pool is 192.168.100.220 to 192.168.100.254), if you turn off 'use default gateway of remote network' setting, the split-tunnel will be working fine.
You do not need to do anything more.
2 If you assign the ip ppol address is different with your inside network,for example : you inside network is 192.168.100.x/24 and your PPTP pool is 192.168.1.1 to 192.168.1.254.
In spite of you turn off 'use default gateway of remote network' setting, you still need maually add static route when you connect with PPTP.
route add 192.168.100.0 mask 255.255.255.0
That is the only way to make PPTP split-tunnel working.
Because PPTP is Microsoft VPN protocol, we can not do too much for that.
Try to use Cisco unity VPN client, that is much handy in split-tunnel funcation.
Please check following URL for split tunnelling part: