Connecting them together will reduce separation on all the outputs. The simple way is to replace the 1K resistors (back by the RCAs) with at least 10K, up to 50K resistors. More resistance gives less loss of separation. The high resistance may reduce the signal, depending on the amplifier's input impedance, and it will certainly reduce the high treble due to the capacitance of the interconnect - but that's not something you need to worry about in a bass amp! The high impedance will make the cable more susceptible to picking up hum from electric fields, so choose an interconnect with a good high-coverage shield.
Instead of the speaker-level filter, you could use an active crossover, or probably even a passive line level one, upstream of the mono amp. That would reduce wasted power in the mono amp. Id you do use the passive speaker-level filter, and if the mono amp has negative feedback (i.e. solid state or push-pull pentodes) make sure there is a load at high frequencies to prevent amplifier instability.
