When you say highs, I assume you mean fullbandwith. As long as you're getting the proper frequency band, probably not significant soundwise, for a sub amplifier as they have a built in crossover and that full range should transfer through to the other amp as well.
That said, I'm still a proponent of having the most uninterrupted signal path, least amount of anything in the signal path. One of the endearing qualities of the SIQ125.4 and SIQ 75.4 that run my entire front stage is that they have zero x-over electronics in the amp, straight signal from my DSP unit to the amplifier - clean, uninterrupted signal path.
Evan running active tweeters from those amps, for low frequency blockers (inline tweeter protection or bass blockers as people like to call them) I opt for a bit more costly metalized polypropylene film caps at $20 a piece over $2 electrolytics. I think that anytime you place things in the signal path it alters something so I would prefer splitting the output prior to input rather than running it though one electrical circuit to another amp.
