It will depend on how you wish to implement.

Do you wish to propagate the indexes onto a dedicated Search server?
What type of integration do you wish between the two SPS servers?
What functionality are you planning  on using?
- Just document-management and search
- Portal also?

Do you wish to integrate certain back-end apps onto the Portal?

So, I guess the main question back to you is..... what do you wanna do?

Give us some additional information and we'll take it from there.