I'm not aware of a splitter made for this purpose. You will probably have to make your own.
The portable Amigas need to be supplied with +5 VDC (volts direct current), +12 VDC, -12 VDC and a common ground (0 VDC) for those 3 power rails. Those voltage rails need to be accurately regulated to not fluctuate under load (+5 VDC cannot droop down to +4 volts). There can be very little ripple or noise on those rails. In addition, those power rails need to able to supply their voltage at some flow rate, current, rated in Amps or milli-amps (mA).
The current an Amiga draws can vary as drives spin up, accessories and expansions are added, etc. If you don't have a meter to measure this demand directly, the rating on the stock power supply is a good rule of thumb. It is usually over sized some percentage to allow for the usual expansions of 1 system (but Not enough for 2 systems!).
If you have the know-how to substitute a power supply of more than double the current ratings (for each individual rail) of the original, then yes, you could run 2 A600s from 1 power supply. Generally, portable Amigas are frugal with power compared to full desktop machines, so larger supplies are plentiful.
I usually run my A500 and whatever side-car expansion from 1 supply, rather than letting the side-car draw power thru the A500.
Caution: if you get this wrong, you can damage everything connected.