Yes, you could save a system image from both PCs to the same external HDD if you have enough space on it if you like.
On you main tower, since you have the paging file and temp internet files on it's second drive, both drives will be included with it's system image by default and will take more space on the external HDD.
You're not going to get much performance increase by having the paging file and temp internet files on the second HDD unless it's faster than the drive Windows is installed on. Otherwise, it's mostly to save space on the drive Windows is installed on.
Hope this helps,