I would suggest you build a simple two channel mixer, feed the electromagnetic input to it via an opamp amplifier, with an input impedance about 1Mohm, and set the gain to be suitable. Feed the piezo via a higher input impedance, 2M2, 4M7, or 10Mohm - either use a suitable opamp, or if the output is truely as high as you say, use a high value series resistor to feed a lower input impedance opamp (making an attenuator). Using the two mixer level controls, you can mix the proportions from each pickup to get the sound you want - or you could use a single pan/balance pot to fade between them instead.
Obvious thing! - don't even think of using a 741, use something a little more modern and better spec, like the TL072 series.