What is the best solution to parallelize non-rechargable batteries for best efficiency and low self discharge
It is not safe to run primary lithium batteries in parallel!  One will drive current into the other and overheating may occur.  The batteries may vent and the electrolytes in these batteries are highly caustic.

Two options:

If you need the combined current, use a pair of schottky diodes, one from each battery to the load.  This will cost you a fraction of a volt, but is simplest.

If you use mosfets to turn them on one at a time, be sure that there is no scenario in which current is able to flow out of one battery and into another.