You can not
run both OSes at the same time on the same Mac, but you can have them installed on separate volumes (partitions or disks) at the same time and run them consecutively.
So what you'd need is a(nother) partition or disk that doesn't already hold a copy of Mac OS X, or which you can overwrite with Snow Leopard. Certain utilities allow you to partition existing volumes that already contain data (like your iMac's HD), but because there's always the risk of data loss it's best to start with a fresh partition/disk. Apart from that, such utilities may not run on Tiger, which would leave you only the extra disk option. (But see
Stellar Partition Manager, with which I'm not familiar.)