Windows 7: External HD for backup

I am considering purchasing an external HD to use as backup for my desktop and my wife's laptop. I have Windows 7 Home Premium 64 bit on my computer and my wife has Vista 32 bit on her laptop. Would there be any problems in having one external backing up both systems or would it be better to just get two HD's?
Thanks

I dont see any problems using on HD for both machines. The question comes in what kind of software are you going to use? If its the default backup items in Windows then you should be ok too. Im sure someone will be along to advise about different types of backup software to use on the machines.
Myself I have one HD and just copy over the MY Docs folder and thats it.

Thanks for the info.
I have Acronis 2009 on my computer. No additional backup software on my wife's laptop yet.
Would you advise making the external into two partitions or just label the backups as his files and her files?
She has the reinstall disk from Dell for her computer so she just needs to back up her files that she does not want to lose.
I installed Windows 7 using the upgrade disk. I did a custom install from Win 7 RC, build 7100. So I will be looking to back up the OS so that I will not have to do a reinstall along with some files.
Again, appreciate your help

I would just create two folders His/Her's no need to cut the drive into two parts unless there is a total need to.

Thanks