what is effect of installing 2 capacitors in parallel on contact breaker in car distributor ?

This must be an older vehicle with Kettering ignition. The capacitor is there to allow the points to open a bit before the voltage on them gets too high. Without it the points will arc and the energy stored in the ignition coil will be lost in burning the points instead of firing the plugs. The smaller the capacitor value, while still suppressing the arc, the better. A larger value will decrease the spark voltage. You can be pretty sure the proper value was selected by the design team.

What is MK - capacitors are sized in uF (microfarads) ?

Installing two identical capacitors in parallel will double their total capacity, therefore their spark-quenching ability (the main reason why they are required). It will not affect the spark, the breaker contacts should last longer, there shouldn't be other effects worth noticing.

However, there is a downside - capacitors are notorious for failing, and sometimes they fail by becoming conductive, therefore shorting the primary ignition circuit. In that case the engine will not start. Although the likelihood of such a failure is rather low, the one of EITHER of the two capacitors failing is twice that value.

I would rather carry a spare capacitor in my car, ready to be installed if failure occurs.

Spark only increases with a high performance set of coils. The capacitor just causes the points to induce the coil to spark. Unless you have thousands of dollars in motor mods you don't need more spark.

How will that increase spark??