Unfortunately that will not work. All batteries in a battery pack need to have the same Ah rating otherwise their Voltages will become unbalanced during discharge and recharge cycles and the difference in Voltages will continually increase until one of the batteries is undercharged and the other one is overcharged to the point of failure. Both batteries need to have the same Ah rating. Both batteries should also be the same brand and same age, otherwise the same unbalanced Voltage problem could occur due to differences between the batteries.
Pete Rohr