Planted Tank Guru
Join Date: Jul 2010
Location: Huntington Beach, CA, USA
If you joined two canister filters of the same type, I would guess that the flow might be slightly higher at the start. But that doesn't mean that it would be worthwhile. One can get far more flow by using the two as separate filters. One of the big downsides to joining them would be the way the first filter would clog sooner. Once the media in that filter is stopped up, any gain would be totally shut down.
Since filters are designed by people who do this full time for companies who do a lot of research on what works, I see very few people who can actually improve the results and can prove it with actual measurements.
As a person with a very strong DIY gene, I'm often tempted to try to improve things but filters are not a place where I mess with the design much. I find far more untested theory on the INTERNET than I find actual proven results with testing to back it.