According to my Naimuniti 2 reference manual, “The preamplifier output is designed to enable an upgrade or additional Naim Audio power amplifier to be connected to the NaimUniti 2 preamplifier”.
Based on the above thread, Bi-amping is not the way to go and that possibility for “additional Naim Audio power amplifier” only exists for a multi-room setup?
The pre-amp output on the Uniti 2 allows you to upgrade from using the internal power amp to using a dedicated power amplifier such as a NAP200 or NAP250.
Yes that is the first part of the copypaste from the manual, “The preamplifier output is designed to enable an upgrade”.
However, the second part says “…or additional Naim Audio power amplifier to be connected to the NaimUniti 2 preamplifier”
I guess in theory you could run the internal amp and an extra power amp to passively bi-amp, but probably not worthwhile, and would only be possible if your speakers allow passive biamping (and still likely not worthwhile!).
Which speakers do you have? It may be worthwhile upgrading the whole system first before considering going active.