No, since you can't tell the algae to go into one and parasite to go into the other, you are basically making the whole system less effective bc half of the stuff will go to the "wrong" one.
The people that say "stick with the slow flow one bc if it can sterilize parasite then it can def sterilize algae" is only partially true. For algae - there is a reason you need to flow to be fast - , bc they reproduce faster, the flow needs to be fast enough so that you can turn the total tank water volume over a few times a hour to ensure maximum effectiveness. For these, the important point is that the flow needs to be just slow enough to sterilize the algal cell in 1 pass, but also fast enough to turn the tank over so that you minimize the "live" ones in your tank from reproducing faster than yiur UV can sterilize. So, if your UV isn't powerful enough, making it slow may kill both the parasites and algae that pass thru it, but it may not be fast enough turnover to keep up with the algae reproduction in your tank.
If you are spending money on 2 UVs, its better to just buy 1 overpowered on instead, bc an oversized one will have enough uw/cm2 to kill the pest under the "slow flow" mode but bc its oversized for your tank, even a slow flow mode will give you enough tank turnover to keep pace with the algae since your tank volume is smaller.
