Portable solar

Tony.
For a regulator to "regulate" it has to be able to sense battery voltage, therefore a second regulator, in series, with the first one will not work. So, NO is the answer.
Is the regulator supplied with the panel a PWM, ie, Pulse width modulated, which effectively just switches ON or OFF to control the charging voltage, ie, pulses/surges, OR an MPPT regulator?
I suspect it will be a PWM reg which is not as efficient as an MPPT.
You could simply cut the wires between panel and original regulator and feed those panel wires to the correct inputs on an MPPT regulator. Preferable if the regulator is as close to the battery as conveniently possible and ensure the lead from the panel to the regulator and reg to battery are a suitable size. Usually needs to be bigger cable than panels provide.
Best results will be obtained if the good size cable starts at the terminals inside the solar panel terminal enclosure. and that sensible size continues for the rest of the run.
An MPPT regulator WILL at times provide MORE amps into the battery than is entering the regulator from the solar panel. Simply the way MPPT works to extract the most energy to deliver to the battery. Cable sizing is to do with minimizing voltage drop along the cable.
So, 1 Reg only. Best an MPPT.
A panel with included reg, appears to be a cheaper purchase brand, could be wrong, if so, I wouldn't worry about warranty as it is hard to extract from those sellers anyway.
I have a couple of "ECO Worthy" 20 amp MPPT regs which are adjustable in their parameters of charging profile and so can be altered to suit different types of batteries AND has a screen to show what is happening.
Nice to have a screen on the reg, but if it works properly you have no control over what is happening. Only advantage with a screen for normal system or portable, is to be able to see the max charge amount while panel pointing at sun or not, and aids alignment until the sun moves, then it is less until aligned again.

I,d get ridof the reg you have now and buy a Victron MPPT with bluetooth. It will be a big improvement for your panels and the bluetooth readout will tell you exactly how the panels are erforming. Warranty....good luck with that. MOST IMPORTANT....Do not have the regulator on the rear of the panels! Mount it as close as you can to the battery. Cut the old reg out and fit anderson plugs to any panel. If you wish to join 2 or more panels buy/make 3 way anderson connectors....cheap.

Agree with Big Fish. Love Victron stuff and the app is easy to use.

All you need is something like this, there are cheaper ones on ebay though
https://www.australiandirect.com.au/Battery-Systems/Electronics-Appliances/KAMeter1