Sure you can... You have 2 possibilities:
1. Install distro1 with grub/lilo, then install distro2 without a bootloader and then edit grub/lilo so that it can boot both of them (but it might be a bit tricky)
2. Install distro1, install distro2 with bootloader, and it should automatically recognise the other bootable partition and add it.
It's a good way of learning how the bootloader works