yes you can. you would have to partition the main drive into two HFS+ partitions and then boot into the tiger installer and install tiger on the second partition. you can use leopard's disk util or ipartition to do this without erasing.
but buying another drive and shoving it into the second bay would be a much easier option and albeit a lot faster as the tiger install would be on the start of the HDD platters and not on the end part if you partitioned your main HDD.