The Aphex unit suggested by Tls49 does look like an excellent solution, if you don't mind its cost.
Regarding the original question about using cable splitters to split four ways, it depends on the output impedance of the sub output of the Meridian (at deep bass frequencies), and on the input impedances of the JL subs and the Velodyne amps. Unfortunately none of those impedances appear to be specified.
If you want to pursue that approach, contact Meridian and ask them what the output impedance at 20 Hz is for the 861's sub output. Or, alternatively, what is the minimum load impedance they would recommend that it be used to drive.
Also ask JL and Velodyne for the input impedances of the balanced inputs of those units.
To assure good results, the combined (paralleled) input impedances of the JL subs and the Velodyne amps should be at least ten times as great as the output impedance at 20 Hz of the Meridian's sub output. Or, alternatively, the combined (paralleled) input impedances of the JL subs and the Velodyne amps should be greater than the minimum load impedance Meridian may recommend for the 861's sub output.
The combined (paralleled) input impedances of the JL subs and the Velodyne amps equals the reciprocal of the sum of the reciprocals of their individual input impedances ("reciprocal" meaning the number divided into 1). For example, if the input impedances are 10K for the Velodyne amps and 15K for the JL subs, their combined input impedance is:
1/(1/10 + 1/10 + 1/15 + 1/15) = 3K, or 3,000 ohms