Combining two antenna's?

I am working on a design/research project at school, on idea being tossed around is two take two input antennas and put them in series with each other to have a higher input voltage. The conceptual plan right now is using two or four patch antennas in-conjunction with each other. I was wondering if we would need an antenna splitter or if we could just put there feeds in series with each other? The purpose of this is in extremely low power RF energy harvesting, and using multiple smaller antennas to increase rectifier efficiency is one of the ideas we are pursuing. I was hoping that it would be possible to use a commerical antenna splitter for this purpose but most of these are matched to 50 or 75 ohms and we will not be using a 50 or 75 ohm antenna or system.

I think you are speaking about a rectenna array (with two elements). Which frequency ?
Possibly you are interestd in this article: http://www2.ece.ohio-state.edu/~olgunu/rectenna.pdf

The problem is that the signal from the antenna is not always in phase at the summing point. If you connect two antennas together then sometimes the signals from the two antennas add and sometimes the signals are 180degrees out of phase with similar strength and cancel out nearly completely.

When you connect two omnidirectional antennas together you usually create one directional antenna with 10-20dB nulls in the radiation pattern.

With patch antennas, if all the antennas are aimed at a distant transmitter, they are not getting different signals due to reflections from objects, the antennas are close together so that different arrival times from the source dosn't make a big phase shift and the coax or transmission lines to the combiner are exactly the same length then it may be an improvement most of the time.

If you have patch antennas pointing in different directions then sometimes the signals will add and sometimes the signals will subtract.