Impedance matching is generally the biggest killer for amps. It's not just a case of adding more and more subs to one amplifier. That will usually result in a fire. Make sure the Jaycar is 2 ohm stable in bridged mode (not likely) if you are running 2 Clarion 4 ohm subs in parallel. Most amps that are 2 ohm stable are single channel mono blocks. If you want to run 2 subs off a mono block amp you will need 2 x 4 ohm subwoofers wired in parallel as a 2 ohm load. If the Clarion subs you have are 2 ohm, they can be wired in series bridged off your Jaycar amp. For power matching you are better off going over board. For example, to run a 300 Watt RMS subwoofer you should be using a 400 Watt RMS amplifier. That's RMS not boss audio "peak" 8 billion watt bulls@#t ratings. That way you can run the sub to it's full potential without worrying about the amplifier clipping or shutting down from thermal overloads.
Alpine, focal, eclipse and AVI amplifiers are among some of the better units as it comes down to a lot more than sheer power to make a real amplifier.
As the saying goes, "oils aint oils" same as amps aren't amps. Check for THD (total harmonic distorion) levels, signal to noise ratio's, damping factors, frequency response, x-over networks etc. Holy poo! I just realised how much I've written. I can go into more detail as to why these features matter if anyone is interested.