[Addressing updated question:]
It is possible that a cheap fiber filter will not reduce airflow (CFM) too much if there is one regular one on the unit and one fiber on the return air vent. It entirely depends on the filter. It will definitely have some measurable effect on CFM. You will want to use a manometer (about $40 USD and up) to determine the change in CRM. Measure with on in the unit before the coil in the supply plenum and one outside the unit before and after you replace. It also wouldn't hurt to take a reading before and after on the return side before the coil in the supply plenum. (see below for illustration) For the latter, you'll need to look up your blower unit's specs to determine its rating. You will still likely want to have your ducts cleaned. See this link for more details.
If you don't want to go through the expence and trouble then I would recommend erring on the side of caution and not install two filters. (Keep reading to see why.)
[Addressing Original question: Use of two filters. (As well as other helpful info, assumes two regular filters at unit. )]
The reasons to replace the filters in central air systems is because they either become too restrictive on airflow (CFM) or they begin to not work as well by letting in the very things they are trying to filter if not replaced overtime. Putting in two filters might temporarily address the 2nd factor but it will have a negative impact on CFM and may, in fact, require that you replace the filters more often.
Assuming it is a PSC motor in your unit, the actual amp draw on the motor will be reduced with a reduction in CFM (1), the net effect of a reduction in CFM from the return air will cause your Central Air Unit more wear and tear as the motor will have to spin more RPM's to move less air throughout the house putting more wear on the brushes (if it has any) and bearings, and using, in total, more power because it is running longer. Also most blowers run the risk of overheating if CFM is reduced too much. The lower the CFM the more the heat the blower unit generates and this reduces life on the blower as well (1). ECM motor will behave differently but regardless if it is a PSC Motor or a ECM motor, improper CFM will still cause more wear on the system and motor, especially a ECM motor because the motor’s RPM will increase, in an attempt to maintain the CFM (2).
Bottom Line: Putting two filters will not make it last longer but may make you have to replace it earlier, assuming it doesn't already reduce air flow too much with just the filters.
See picture above: Reducing CRM on the return air, will reduce the force of the air coming out of vents as well, making your entire system have to work longer to heat or cool your home to the desired temperatures. Overtime this will put more strain on your entire central air unit including your blower, cost you more money on your utilities, and may cause premature failure.
Assuming your home also has an A/C condenser, there is also the risk of destroying the compressor. When there is not enough airflow through the evaporator the suction pressure is below normal because the refrigerant flowing through the evaporator picks up less heat than normal resulting in lower pressures. Liquid could be returning to the compressor through the coolant lines which overtime will eventually cause the compressor to fail (3). These compressors are not cheap and neither will the labor to replace! Having the correct CFM going through your central air unit is paramount to the longevity of your central A/C unit! CFM will make or break a Central AC!
Bottom Line: Putting two air filters on a central air system will only have the net effect of producing a higher utility bill and reducing the lifespan of your central air unit.
In short, the use of two air filters is just a BAD idea! I would strongly recommend against using two filters at once, especially if it is just by reason of convenience. You'd be much better off resolving the reason why you have Pet hair in you unit. I'd recommend having your ducts cleaned and if your return air is near the floor, then make sure to keep that area a clean as possible. In any case, replacing these filters is going to be much better than premature failure of any part of your system.