It's always a cost effectiveness thing when it comes to photovoltaic panels. Regarding the second question I'm not certain, the panel should be always producing some energy from the light it gets hit from, but the voltage needs to be beyond a certain threshold to be used. And since the voltage depends on the light intensity the positioning of the panel affects a lot the total power that can be actually used in a day. Lenses(?) and mirrors can improve the production but they have to be pointed to the panel, and since the sun moves they have to move too, and the mechanisms to make them move make the whole thing more complicated and probably expensive than simply buying a second panel. That said solar towers exist (look them up they look cool) but they don't rely on photovoltaics but instead use the heat to boil water and pass the steam through a turbine to produce electricity.