@GrM - I think you missed the OP’s qualification for the setup in that they are using an android tablet for a head unit that doesn’t have its own amplifier. At least, that’s what I assume they mean by android tablet instead of android OS HU. Thus, the need for a separate amplifier and the preponderance of powering four speakers off two channels.
To the OP - yes, you can do that. As
@miniSQ mentioned, it depends on the amplifier and impedance rating (ohms) of the speakers. Typically, car speakers are rated at 4 ohms and most two channel amps are rated down to 2 ohms. If it is a bridgeable amplifier then it is certainly 2 ohm stable in stereo. If you wire the front and back speaker in parallel to the amp, and they are indeed 4 ohm speakers, it will se a 2 ohm load and be perfectly content. If you wire them in series to the amp, it will see an 8 ohm load and also be perfectly content, albeit with half the power output of its 4 ohm rating.
Wiring in series is the safest option and will almost certainly work, but with reduced power output. Wiring in parallel will work give the conditions listed above are met. Either way, you won’t have the ability to fade front to rear. If you needEd to permanently tone done the rear speaker level, you could use a simple LPad (two resistors wired correctly) to do so. But that’s prolly more effort than you want to put in to the project.
