Hi Ymsk,
1) Theoretical, it is possible to connect the two converters in parallel to double the maximum load current. However, some modifications are needed to maintain load sharing and stability between the two converters and reduce input and output ripple, also the 2 devices might not be balanced very well and synchronization with 90 deg phase shift for example is also needed, which makes this option less practical. For high current applications, the ADP1850/1 may be a good fit.
2) Both ADP2114/2116 and ADP2323/2325 (and any other converters) can achieve the same noise level when using filtering ferrite beads at the output of the converter, yet it is hard to design the 2nd stage filter with high output current(>4A) and keep the loop stable.
3) For the negative output, we have some power parts can be configured as inverting Buck-Boost ( ADP230X, ADP238X, ADP244X ) and SEPIC-CUK( ADP161x, ADP1621). For low noise application, ferrite bead filter or LDO is needed as well.
All the design tools are downloadable at www.analog.com/ADIsimPower.
Best Regards,
Hong