Current splitter/divider in nA/pA range possibilities
Here is a cheap approach, but you need access to a foundary with long-channel FETs. The purpose of using a foundary is the access to layout of matched (interdigitation of alternative strips) FETa and FETb, for matching during implanting and, later, thermal matching during operation.







simulate this circuit &ndash; Schematic created using CircuitLab

How fast can a current splitter function? Or, how much charge is needed?

Assume the FETs are 1U by 1,000U each. Area of 2 FETs is 2,000 (micron)^2 which is about 4 picoFarads, in 0.6 micron process (140 Angstrom oxide). To change the voltage by 0.005 volts requires

Q = C * V

dQ = C * dV   [assuming C is constant, and some FETS do vary near Vt]

dQ(coulombs) = 4pF * 0.005 volts = 0.02 picoCoulombs

At 1pA, this allows about 1pA/0.02pC or maybe 50 cycle-per-second operation. 

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So what to do? buy tiny matched same-die JFETs, tweak the gate up or down by a few milliVolts on one of the FETS, so the current splitting is accurate.