Do front and back porches for digtal video really matter?
Of course timings matter even if using DE only. You must read the datasheets if there is a common range you can use for both, or if the current settings are good for both displays. Many displays ignore HSYNC/VSYNC and only use the DE so if both displays are only using DE then you should be fine. Better yet if the pixel clock, horizontal total and vertical totals are according to standards so you get a standard line rate and refresh rate.