Converting to a dynamic disc will probably not be a problem. What you are doing is a pretty simple operation. People run in to problem with dynamic disks in more complicated scenarios.
A better solution in the long term is to create only one volume per VHD. That way when you need to expand a drive in the future, you don't run in to this problem. To do that now you would need to shut down the VM and use third-party tools to copy the second volume out into its own VHD. You might run in to a problem here if you convert to a dynamic disk, since most of these utilities don't support dynamic disks.
A better immediate solution might be to enable dedupe on the volume, and kick off an optimization immediately. This might require a reboot (if dedupe is not already installed) but a quick reboot might be a good compromise between using a dynamic disk and not rebooting, and the extended downtime needed to implement "one volume per VHD". Plus you don't run the risk of not being able to implement "one volume per VHD" because you converted to a dynamic disk.