The use of "Tee" or "Y" connectors is an effective way of the connecting two oxygen concentrators to increase the flow-rate.  This can achieve the magical 15lpm that match the maximum flow rates on oxygen cylinder regulators/flow-meters.  However, it is NOT recommenced to Tee together the output of a dual-flow concentrator.  The Jay-5 pictured here is designed for 5lpm and if the output is increased beyond 5lpm the purity will drop.  When both sides are connected and both flow-meters are set at the maximum setting of 5lpm (giving a total of  10lpm), the purity is less that 60%. 

Recommendation:  To prevent misuse of concentrators with dual flow-meters, training is not sufficient.  The best option in our opinion is to convert these concentrators to single flow.   We do this by turning the knob on the left flow-meter fully off, capping the oxygen output, and placing a sticker over the flow-meter labeled "Not In Use".  This essentially turns it into a single flow-meter concentrator.