Question isn't clear. What do you mean with "sum"?
Differences are pretty complicate but, I attempt the silly thing to describe it in 2 sentences.
If you place DC sources in series, you get the voltage equal to the sum of all the sources' voltages. If you place them in parallel - given a diode between each positive lead, to avoid short circuits (author's note: thanks to the user in the comments) - you get the voltage equal to the higher source voltage. About current: when they are in series they all supply an equal amount of current as far as they all can supply the requested amount; when they are in parallel instead, the one with higher voltage supplies the load alone until it lower the voltage (because can't sustain that power request) or fail.
AC is similar, actually more complicated. To start you MUST put the 2 sources in sync, otherwise any bad thing can happen. Usually what you do is checking the zero crossing point and use it to sync the two AC sources...
You probably see the AC being easier because solar inverters usually have connections to sync when connected in parallel.
That said, I invite you to start reading the basics and use your application as a learning tool, before attempting any experiment. Because it can be expensive and dangerous to hands on without the basic theory.