You will be straight into other potential issues pairing an new 100Ah battery with a "used" 60 Ah one; the general advice is they have in all respects to be twins.
Also hoist in two other bits of info;
1) Even the very best LA batteries, and those designed for that duty can withstand the punishment that comes from deep cycling, and with what inverters open up, very high current draws, often encountered replicating "mains" supplies.
2) Even the very best [read very expensive] deep cycling batteries start to really suffer if taken lower than 50% SoC [state of charge].
So the set up if it has 160 Ah [out of the box capacity] batteries has an occasional usable 80 Ah. That is at 12 volts [give or take], so 960 kWh, be that at 12 volts or mains with a lossless inverter. Note that is just under one unit of mains power, very little indeed.
And as said do it routinely and any battery is going to fail, some way earlier than others.
Your requirements if not your budget is better served on several counts by ripping every thing out and going to the Lithium technology route