It's it is a general rule to have one backup methodology or product if that is possible. But it is also true that it is not always possible or even advisable in any given situation.
The perpetuation of virtualization is a perfect example. For too many years, the virtualization backup capabilities of the leading backup products could be described as anything but "leading." This led to an entire sub-category of products designed specifically for backing up virtual systems. Unfortunately, these same products have generally eschewed support for physical systems. Since most customers have both virtual and physical servers, this ends up requiring them to purchase and manage multiple backup products.
Another reason for a customer purchasing multiple backup products may be they have an application that requires the capabilities of a CDP or near-CDP product. These products can provide very low-impact backups and extremely fast "instant" restores, so many companies have started using them for their applications demanding tight RTOs and RPOs. But they're not necessarily ready to replace all of the backup software they've already purchased in favor of this new way of doing backup. This again leaves them with the requirement to manage multiple products.
There are many challenges with using multiple backup products, the first of which is that they all behave differently and should be configured differently. One of the most common ways to make a backup system work poorly is treat it like a different product. TSM and NetBackup couldn't be more different than one another, but many people move from one of these to the other -- and still try to configure the new product like it is the old product. The solution to this is simple: get training on the new product and consider hiring -- at least tempoarily -- the services of a specialist in that product to make sure you are configuring it the way it likes to be configured.
Another challenge is that each product reports on how it is performing in different ways. They may use different metrics, values, and terms. They also use different delivery mechanisms. One may use email, where another may use XML or HTML to report backup success or failure. The key here is to use a third party reporting system that can collect and analyze the various product and normalize them into a single reporting system.
Avoid having multiple backup products when you can. When you can't, employ significant amounts of training and look into a third-party reporting tool that can handle all of the products you are using.