Multiple UVB bulbs won't necessarily intensify eachother, but would create a larger area with basking levels of UVB. If the basking light and tube are each set at an appropriate distance for their output, then it should be safe. I usually shoot for 3-6 UVI over the basking area. They can tolerate more than that but don't often want to. You can create areas of higher intensity available via branches or similar if you want to, then just observe behavior and adjust based on how they use the space. Another key thing to note is that you'll want areas of little to no UVB as well, so that they can retreat from it completely if they want to. They are good at regulating their exposure, so if they have options available they'll move around to get what they want at any given time.
