Implications of using two data sources, two gateways, two dataflows and ONE dataset?

Hi, that is ok and it will work. There is another approach you could use. The dataflows can help building another layer from a transformation purpose that can take the data once and hold it. However they are note completely necessary unless you want for sure to keep that layer.

The other alternative would be to turn one of the gateways as cluster of gateways, then re install the other one of the gateways in order to make it part of a cluster. Having both of the in a single cluster will make Power Bi believe that it's one gateway that knows the two sources. Then you can start working and connect with Power Bi Desktop (be sure your machine can access both of the sources). After publishing, Power Bi Service will be able to configure the dataset for both sources on a single gateway.

I hope that helps,

P/D The best performance for data manipulation will always be in a db engine.