The D5's, as they come, are not as well-optimized performance-wise as they could be (not to mention they are, quite frankly, ugly as sin). While it is certainly 100% possibly to simply plumb one pump into another, the dual pump tops makes everything easier, provides the performance benefits of an aftermarket top, makes the pumps look a LOT better, and makes mounting much easier. You can even, with a dual D5 top, connect a cylinder reservoir directly to the dual-pump top's G1/4 inlet via Bitspower Male to Male Rotary Coupler Fitting (and 10/15/20/25/40/50mm extension, if you want to give some distance between the res and pumps).
This one's my favorite:
Bitspower BP-2D5TOPP-BK Dual D5 Mod Top (POM Version)
You can pair it with the following to add a reservoir and whatnot:
Bitspower Dual/Single D5 Top Upgrade Kit with 150mL Reservoir (Black POM Cap)
And if you really want to make them pretty, you can add Mod Kits, such as the following:
Bitspower D5/MCP655 Mod Kit - Matte Black Finish
Other colors are available as well....
D5's, while not my personal choice in pump, sure are customizable!