Short answer, no.
More importantly if you are using 4 speakers, one of which is not complete, and 1 amp only, your impendance loading is going to be off. If the speakers have a 4 ohm load each and you hook them up in parallel (usual way of doing it) then you are loading your amp with 2 ohm, which in the lower regions is gonna be like shorting the amp. A surefire way of getting yourself in trouble.
Any two-way passive speaker (which I understand is what you have) will have a passive crossover in it to get the high frequencies to the horn and the low/mids to the 15″. Theoretically you could disconnect the horns (well one is missing already) and they would only play the lows/mids.
Again, this will probably throw off your impedance loading and possibly mess up your amp. Another thing is that subs play lows BELOW the frequencies of the low/mid in fullrange systems. So while getting extra low/mid, you are not actually gettin sub-low.
My money would be on using one set of speakers and forget about subs and added bass. Alternatively, if it’s a paying gig, using a bit of the money and renting a powered sub with high pass filtering. Hook up your amp/passive speaker to that and you have a good 1 sub/2 top system going.
Good luck.
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