Within an electrical circuit that has multiple parallel branches the current will be split between these branches. Following Ohms Law the current is inversely proportional to the resistance. In other words, if two or more resistors (e.g. tissue) are in parallel, the input current that enters the combination (here from the stimulus isolator) will be split between them in inverse proportion to their impedances.
As a result, if the impedances have the same value the current is split equally. However, since the resistivity between the different pairs of electrodes usually is unknown, one can't predict how much current will reach each tissue sample.
If you need to perform electrical constant current stimulation simultaneously on multiple channels, you would need a multi-channel constant current stimulator. Please ask your local ADInstruments sales specialist for advice.