One of the controlling factors for the depth of investigation in a groundwater measurement with surface NMR is the available excitation current. Increasing this current is generally a difficult and costly engineering challenge. In this paper, we experimentally demonstrate a different approach towards higher excitation currents. We simultaneously operate two surface NMR instruments with overlapping transmitter coils, which effectively turn them into one instrument with a maximum current twice as large as a single instrument. An analysis of the NMR excitation pulses shows that the combined pulses are very stable with negligible amplitude and timing errors. A field example is provided where data from a single instrument and from the dual instrument setup are compared. The paper contains a discussion on how this approach can be extended to novel surface NMR transmitter strategies.