I would remove the second impeller, removing the first impeller means the inlet flow to the second impeller will have to travel thru the 1st stage volute discharge opening which leads to the second impeller inlet, this will have the effect of increasing inlet losses.
However, I would check with the manufacturer or agent for their advice on this.
You shouldn't remove either impeller stage if you can avoid it because you may be faced with difficult shaft radial or axial loading and rotordynamics rebalancing problems. The first stage is more important because it is usually designed to meet the cavitation design needs whereas the 2nd stage impeller may not provide features for cavitation control. If you are presently developing twice the needed head, then you are probably operating at a flowrate that is far below the best efficiency point (bep)flowrate but you need to know exactly how far below bep. If operating below manufacturer's recommended minimum flowrate, flow instability is likely to lead to high radial thrust loading and possibly excessive shaft vibrations. If you don't see excessive vibrations then you may be operating below the unstable flow range where continued operation without damage may be possible. Typically, worst instability flowrates are between 30 and 70% of bep flowrate.