assume I would need to use two functions generators
You would not need to go through the trouble of buying signal generators, rather construct your own oscillators as they are easy to make even on a breadboard, depending on what is your frequency of interest.
If you are looking for fixed frequencies for your signals, a phase shift oscillator would be good since it does not need gain control to generate a sine wave. If you have different frequencies, you can make a Colpitt oscillator and tune it with varactor didoes. You can find all the schematics you need with a quick google.
what sort of circuit would I need to construct?
What you described in your question is a mathematical operation of adding 2 voltages at every given time, pretty much how super-position works with waves. So for that you need a circuit to add your signals together. The easiest way, as Marcus mentioned is an analog adder with an op amp. One thing to make sure, however, is that the op amp or what ever component you will use has enough driving capacity to source/sync the amount of current you need without exceeding limitations (e.g. max output current, max power and ...).