You can dose KNO3 & KH2PO4 together and time of day doesn't matter. They will stick around until they are used up.
Same with Flourish.
The only two fertilizers that you have to be careful of adding around the same time are ones that have a high iron concentration and a high phosphate concentration. This can form a insoluble iron salt that will precipitate out in your water column.
I had not heard that with excel , but no longer use the product so am not the right person to ask.