Two 12v batteries in parallel 7ah and 20ah
If you simply parallel connect the two batteries for discharging, then the combined Ah capacities do not add. As current is drawn from the paralleled batteries, the voltage of the 7Ah battery will typically drop more quickly than the 20 Ah battery. This will typically cause the higher voltage 20 Ah battery to begin charging the 7 Ah battery. Whichever way the current draw occurs, the action of one battery charging another battery wastes energy in the form of heat.

Since part of the energy of one battery is being used to charge the other battery, the combined capacity will likely be more than 7 Ah but less than 27 Ah. The exact answer is not known without an extensive analysis of the load characteristics and the specifications of the batteries.

By adding some electronics to the system, it may be possible to realize an overall higher Ah capacity compared to simply paralleling the batteries.