The paper discusses basic relationships, an example of application and an outline of the state of research on the properties of unconventional measuring circuits with two current sources (2J) or two voltage sources (2E). Sources 2J are connected in parallel, 2E sources -in series to opposite branches of a four-impedance loop with two outputs from its diagonals. Formulas for output voltages and conditions of balance of both outputs of double-source direct current (DC) bridges are given. For double-current bridges, these are the equality of the products of resistances of the arms adjacent to the terminals of each output, and for double-voltage bridges – equality of the sums of arm resistances. These bridges, when both outputs are balanced, allow for the accurate measurement of resistance (conductance) values in their arms. The output signals depend on linear combinations of the increments of arm resistances from their values in equilibrium states. As an example of application, the simultaneous measurement of strain and temperature changes with one differential strain gauge is considered. The state of theoretical work and experimental research results of prototype designs of dual-current DC bridges and measuring devices and laboratory stands with their application are also shortly characterized. The state and directions of theoretical studies and experimental verification structures of measuring systems two-voltage DC bridges are briefly outlined. Two-source bridges of alternating current AC and of other waveforms have a huge variety of possible solutions for measuring two or more parameters. A bibliography of publications on dual-source bridges is also included.