You can considere as another portal system. It depends on your h/w capacity. If you have sufficient CPU and memory you can install on the same host. Please consider the following points before installaling into the same host :
1. Future upgrade plan - In case of parallel upgrade ABAP and Java will create separate shdow instances - so you need more memory and CPU.
2. If you are using ESS/MSS or heavy portal uses - please use separate host
3. For light uses - you can go with same host.
Thanks,
Dipam