I would forget the hi-level signal route, unless your head-unit does not have RCA outputs.
Most H/Us will at least have 2 RCA outputs, one for "front" and one that may be labeled "rear/sub" which means it can be set to run either an amp for speakers or a sub.
Run one set of 2 chnl RCAs from the "front" output of the head-unit to one amp and run a set of 2 chnl RCAs from the "rear/sub" output of the head-unit to the other amp. You may need to split the 2 chnl RCAs at the amp and plug into all four RCA inputs in order to get all 4 chnl to play. So you would need 4 splitters.
The amp's instruction manual will show you this as well.