Hi,
Microsoft generally doesn't recommend this sort of thing, as it will make it difficult to target these installations with vendor updates (i.e. minor upgrades, patches, etc.).
Can you provide some details as to what the end result you're going for is? One method of handling this is a bootstrapper executable that just silently installs both MSI packages using some custom code--pure InstallScript projects are particularly well suited to this, since they are so configurable.