Things to think about:
1. Trying to create a single installer with both bits of software embedded is going to approximately double the amount of bandwidth you need to use to deploy the apps. They then need to be extracted which doubles the amount of free disk space required.
2. You will need to check whether any license conditions are being breached, as you will be deploying two instances of the application to a machine even if only one is being installed.
3. There is no "off the shelf" wrapper code to do this so you will need to create something yourself. It will need to start out as a 32 bit application as this can run on both platforms, but you will also need to ensure that the 32 bit process is capable of correctly starting a 64 bit installation, and not being a programmer, I'm not sure how easy it is to force execution of a 64 bit installer in a 64 bit process thread from a 32 bit process.
4. The process of creating and testing each dual installer will add time and cost to your overheads.
Frankly, I think you should accept the initial overhead in creating and maintaining separate groups for 32 bit and for 64 bit applications, as the long term simplicity and cost/testing savings are incontrovertible.