Splitting line level out from preamp, versus using 2nd set of outputs

Wouldn’t you want a splitter with a female to two males?
Easy to find, like this one (link), but there are many for less money.
Typically, you would connect the splitter directly to your preamp and run the other two cables from there. I have done that to run a sub.

That should be fine. It’s hard to imagine the extra connection point and IC would penalize this enough to notice, unless psychological. 

So you’re using 3 sets of ICs to ensure symmetry between the 2 paths. That’s perfectly fine, but if it were me I’d probably look for an adaptor to work more like a "loop out" from the 1st amp, so I only need that 2nd IC set to hit the 2nd amp (if positioned close enough). The loop-out is used quite commonly on electrostatic headpohone amps (Stax) - and here, I’ve not noticed a penalty from using the loop-out. 

Of course you’ll find folks who are obsessed about the connector quality, and these can get quite expensive on the high end - I’ve been there in the past; now I don’t care so much. 

When "stacking" cables in series, with different brands or makes (especially copper vs. silver), the sound tends to be "additive" in the traits of the 2 cables. Like tube rolling. Which again, is generally fine IMO. IC sonic differences are generally pretty subtle to me, with exceptions for very long runs or specific slots like phono that can be sensitive to capacitance. 

Are you bi-amping?

A splitter made the sound worse for me. When I needed this same feature (amps), I purchased the CODA 07x preamp which has 2 sets of XLRs. This sounded much better than a splitter.

In my case, 1 amp drove speakers and the other amp dove my RAAL SR1a earphones.

Today, I use a Schitt Kara F preamp to do the same thing.

i split single outputs into an amp and a subwoofer. im very pleased with it. 

The OP is trying to avoid buying another 4M cable from the sound of it.  The Y connector should 1M>2F with the male plugged into the amp driving the high frequencies, and the short cable into the low frequency amp, assuming passive biamplification is the objective.  A preamp with dual outputs effectively has a splitter built in.