Cost may be the biggest reason not to place the two on an 8-way server. There are a lot of licensing options, but in general you will be paying for both engines. The published Enterprise Editions of both products on an 8-way server is roughly $480,000 US. http://www.microsoft.com/sql/evaluation/compare/pricecomparison.asp

In addition to the cost of the software, hardware are not linear on large servers.

Operations (backups, failover, recovery, etc.) become increasingly complicated as you have additional procedures that must be documented, followed, and monitored.

Of course your customer can place both database enginges on one server, but is it really a good option?