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It depends at which frequency is the signal.
If it is in the low Rf band ( less than 1 MHz ) an OP-Amp will solve the problem; in the mid band ( 1 to 400 MHz ) a piece of cable is good; above 400 MHz a piece of FR4 is good too.
If the signal is broadband ( and you can loos e 3-4 dB ) Mini circuits has power splitters with one out at 90 degrees shift.